P0128 Most Common Causes? (Professionals recommend)

The most common causes include: Thermostat That’s Stuck Open – The most common cause of code P0128 is a leaky or stuck engine coolant thermostat. Faulty Engine Coolant Temperature Sensor – Faulty wiring or a bad sensor can also lead to a P0128 code reading.

What can cause P0128?

What Is The Cause Of Code P0128?

  • Stuck open thermostat (most common)
  • Missing Thermostat.
  • Faulty coolant temperature sensor.
  • Faulty wiring for coolant temperature circuit.

How serious is P0128 code?

The P0128 code is set when the engine control module (ECM) determines that coolant temperature is lower than the original equipment thermostat’s opening temperature. To put it another way, the engine is running too cool or the temperature does not increase quickly enough.

Can low coolant cause P0128?

Low engine coolant can alter the engine running temperature enough to signal trouble code P0128. Your intake air temperature sensor, coolant temperature sensor and coolant fan could also signal this trouble code, so these should be inspected after you look over your thermostat and coolant level.

What does P2181 code mean?

Diagnostic trouble code (DTC) P2181 stands for “ Cooling System Performance.” It is triggered when the powertrain control module (PCM) detects that the coolant temperature in the radiator and engine is outside of the normal range (too hot or too cold).

Can I drive with P0128?

Can I drive my vehicle with a P0128 trouble code and illuminated CEL? You may drive your vehicle with a P0128 trouble code. You will not notice any issue except for maybe the temperature gauge never reaching normal levels, and possibly the Check Engine light being illuminated.

What does the code P0171 mean?

What the P0171 code means. The P0171 OBD-II code means that, on the first bank of the engine, the fuel system is running weak or a vacuum leak exists near this side of the engine. A lean condition occurs when the engine either receives too little fuel or too much air.

What is thermostat rationality?

The thermostat rationality diagnostic runs during a drive cycle that starts with the coolant temperature below 140°F. This diagnostic looks at the how cold the system is at start up, what the inlet air temperature is, and the amount of fuel burned during that drive cycle.

What does ECT sensor do?

A coolant temperature sensor (CTS) (also known as an ECT sensor or ECTS (engine coolant temperature sensor) is used to measure the temperature of the coolant/antifreeze mix in the cooling system, giving an indication of how much heat the engine is giving off.

Can low coolant cause a misfire?

Internal coolant leaks can foul a spark plug and cause a misfire. Unfortunately, it also means that drivers will run a vehicle with a coolant leak for several thousand miles while the plug becomes slowly fouled.

Will check engine light come on if thermostat is bad?

the most noticeable issues occur when the thermostat gets stuck in either the open or closed position. a malfunction can result in a trouble code, generated by the engine’s computer, which can turn on your check engine light.

What causes low engine temperature?

The most common reasons for the temperature to remain low include: A thermostat stuck open. A bad engine coolant temperature sensor. A locked fan clutch.

Can a stuck thermostat fix itself?

You can either replace the valves with new ones or work the valves back into a position where they can move up and down more freely. Replace the thermostat. This is the last thing you want to do, but if the hoses do not show a blockage or coolant hindrance then the thermostat itself may be malfunctioning.

What does coolant temperature low mean?

Posted on 3/17/2021. A blue engine coolant temperature light only indicates one thing: that your coolant, or antifreeze, is too cold. In fact, the temperature is so low that it is possible for your radiator to get too cold, thus resulting in your engine freezing up and not being able to start.

P0128 Most Common Causes

It is defined as follows: A Po128 difficulty code is a general OBDII trouble code and is defined as follows: Thermostat for the coolant (Coolant Temperature Below Thermostat Regulating Temperature)

Read further: P0128 Most Common Causes? (Professionals recommend)

What does a P0128 mean

The two most typical reasons of a P0128 are a faulty thermostat or a faulty engine coolant temperature sensor, both of which are listed below. The great majority of the time, the problem is caused by a faulty thermostat that opens too rapidly.

How the ECM sets a P0128 code

The ECM is aware of the temperature of the engine coolant. After you start the engine, the engine recognizes the RPMs and fuel mixes you have selected. On the basis of those three readings, it predicts that the engine will achieve a specific temperature within a specified time interval. If your engine does not achieve that temperature within the specified time frame, the -ECM will set a P0128 fault code in the vehicle’s computer.

The thermostat is supposed to stay closed at cold startup

The engine control module (ECM) is aware of the temperature of the engine cooling fluid. Immediately after starting the engine, it understands what speed and fuel mixture you are running. It predicts that the engine will achieve a specific temperature within a specified time range based on those three data. It is possible that your engine won’t achieve that temperature within the specified time frame, in which case the ECM will issue a P0128 fault code.

The ECM can also set a P0128 if the engine reaches operating temperature but can’t maintain that temperature.

That might be caused by a thermostat that is jammed open. Engine coolant is at a low level. A little leak in the cooling system stops the system from reaching its maximum pressure. The cooling fan is not working properly (running too much) Sensor for coolant temperature (ECT) that is not working properly The intake air temperature (IAT) sensor is faulty.

Most common DIY mistakes when fixing a P0128

If you take your automobile to an auto parts store, there’s a 99.999 percent probability that the salesperson will inform you that the engine coolant temperature sensor has to be replaced Coolant temperature sensor. Sure, it’s an inexpensive component with a simple fix,’ you’ll think to yourself. So, yes, I’ll go ahead and do it.’ In the vast majority of situations, you will have wasted your time and money. A defective thermostat is the single most prevalent cause of a P0128 error code. Why? Because it is a mechanical device, and there is no precise issue code that will tell you how it is performing, it is difficult to diagnose.

To be sure, the ECT and IAT sensors may occasionally be out of specification, but if they are that far out of specification, the ECM will often set a fault code for those sensors.

How to diagnose a P0128

To begin, check the level of coolant in the system. If the level is low, top it off. The most straightforward method of diagnosing a P0128 is to use an infrared non-contact thermometer. Aim the thermometer at the thermostat housing and take note of the temperature. It’s best to start with a cold engine. The temperature displayed on the thermostat should be close to the surrounding environment. Start the engine and keep an eye on the temperature readings. It will increase slowly at begin, but once it gets rolling, it should accelerate dramatically.

A high-quality thermostat—only STANT or OEM will do—must be installed in order to correct the problem.

Ask me how many of them I’ve had to replace over the years!

Compare the ECT reading with the readings from your thermometer. If they’re within striking distance, continue on to checking the cooling fan operation. Rick Muscoplat was born in the year 2012. Rick Muscoplat posted a blog entry on

P0128 – Meaning, Causes, Symptoms, & Fixes

The temperature of the coolant thermostat is below the regulating temperature.

What Does Code P0128 Mean?

The engine control module (ECM) of a vehicle keeps track of how long it takes for the engine to achieve and maintain the proper operating temperature for the vehicle. In order to maintain the efficient air-to-fuel ratio of 14.7:1 when operating temperatures are attained, the powertrain control module instructs the fuel system to enter a ‘closed-loop,’ in which readings from the oxygen sensors are utilized to maintain the correct engine running temperature. The majority of ECMs require that the engine coolant temperature sensor record temperatures greater than 160 degrees Fahrenheit within 15 minutes of the engine being started.

If one of these two requirements is not satisfied, the PCM will make a note of it.

What Are The Symptoms Of P0128?

  • The Check Engine Light is illuminated
  • The idling speed is higher than normal. Fuel efficiency has been reduced. The temperature gauge has dropped to an exceptionally low level.

What Is The Cause Of Code P0128?

  • The most typical problems are: a stuck open thermostat
  • A missing thermostat
  • A faulty coolant temperature sensor
  • Faulty wiring for the coolant temperature circuit
  • And a faulty coolant temperature sensor.

How Serious Is Code P0128? – Low

The most likely source of the problem is a jammed open thermostat; however, thermostats are often constructed such that when they fail, they remain open, enabling full flow of coolant to safeguard your engine. To begin, check your coolant level to ensure that it is fully charged before proceeding with the operation of the car. In the event that your engine overheats, it will result in engine failure.

Code P0128 Common Diagnosis Mistakes

Many people just replace the thermostat without investigating all of the probable issues. Check the cooling system for rust deposits or a combination of various coolants to see whether it needs to be serviced. When changing the thermostat, make sure to cleanse the engine block and radiator thoroughly to avoid deposits from becoming lodged in the new thermostat.

Tools Needed to Diagnose:

Difficulty in Diagnosing and Repairing the Problem (1 out of a possible 5)

  1. Check to see if P0128 is the only code present on your car by scanning it. If there are any additional codes present, they must be dealt with first. Check the level and condition of the coolant. It is possible for the cooling system to get clogged due to excessive corrosion and poor coolant quality, or for the thermostat to become stuck. Remove any corrosion from the cooling system by flushing it with fresh water and replacing the coolant. It is necessary to refill the coolant system if the level is low and to check for leaks. An electronic multimeter may be used to examine the coolant temperature sensor. The ohm measurement will fluctuate in response to changes in temperature. If the ohm reading does not change with temperature, the coolant temperature sensor should be replaced, or the sensor’s wiring should be repaired if it is damaged. The most typical cause of P0128 is a jammed open engine coolant thermostat, which may be found in many vehicles. To diagnose this, simply feel the radiator pipe and see how hot the coolant temperature is as the coolant begins to flow through the radiator hose
  2. This is a straightforward procedure. It is important to note that you should use extra caution when performing this task since you might get burnt! The hose should only be somewhat heated until the thermostat is activated. As soon as the thermostat is activated, hot coolant should begin to flow into the radiator line, quickly warming it up. if the radiator hose warms up slowly, the thermostat may be jammed open or may be opening prematurely, and it is necessary to replace it.
  • Replace the thermostat, as well as any gaskets and coolant that are worn out. Remove all of the air from the cooling system. Test drive the vehicle and keep an eye on the coolant temperature. Look for any leaks.

Estimated Cost of Repair

One or more of the remedies listed below may be required to resolve the underlying problem associated with error code P0128. The estimated cost of repair for each feasible repair includes the cost of the essential components as well as the cost of the labor required to complete the repair, if any.

  • Thermostats range from $220 to $250
  • Coolant temperature sensors range from $140-$190.

Ultimate Car Repair Report

This free study offers useful information for all drivers, regardless of whether they plan to do their own repairs or seek the services of a local mechanic. Download The Savvy Motorist Report right now to learn more about: Six-step checklist to obtain the cheapest price on repairs, eight mechanic scams to avoid, and how to rapidly diagnose a check engine light on your smartphone, among other things

How to Fix P0128 Code ❤️ “Coolant Thermostat Temperature Below’

If you’re looking for information on how to fix the P0128 problem, first make that the error code is there by utilizing an OBDII scanner. After you’ve determined the source of the problem, you’ll need to check two components: the ECT sensor and the thermostat itself. If any of these components becomes defective, you must replace them immediately. Alternative to this is to have a professional mechanic evaluate the car for more complex testing before driving it. Automobile repairs are EXTREMELY EXPENSIVE.

  1. While it may appear that high temperatures are detrimental to any engine, your engine nevertheless need a specific degree of heat to run correctly at its optimum.
  2. This page has been written specifically for you in order to help you understand what is causing the P0128.
  3. In addition, we will discuss the key causes, symptoms, repair choices, and a general estimate of repair costs to assist you in resolving a P0128 error code problem.
  4. The P0128 code is defined as ‘Coolant thermostat temperature below the regulating temperature’ according to the standard.
  5. The following part will provide you with some basic information about the cooling system as well as the optimal engine temperature in order to help you better comprehend this error number.
  6. Despite the fact that temperature is widely recognized as one of the most dangerous enemies of any combustion engine, the engine must nonetheless achieve a particular temperature in order to function effectively.
  7. The cooling system in your car is the major component responsible for maintaining engine temperature regulation.
  8. Occasionally, when the thermostat fails, the engine will be exposed to the coolant all of the time, preventing it from reaching the optimal operating temperature.
  9. You’ll notice a drop in your vehicle’s overall performance as well as a considerable reduction in its fuel economy when the P012 code is activated.

Your vehicle’s engine will not function correctly when the P012 code is activated. What are the most common causes of the P0128 error code? When your check engine light activates as a result of a problem associated with a P0128 code, the problem is most likely caused by one of the following factors:

  • A thermostat that has become jammed open
  • It might be a faulty coolant temperature sensor. Wiring in the cooling system circuit that is not working properly
  • A thermostat that isn’t working
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What is the best way to tell if it is the P0128 code? Symptoms associated with the P0128 error code Early detection of a problem associated with the P0128 code may be accomplished by keeping an eye out for the following signs and symptoms in your car.

  • The check engine light has been illuminated. Fuel efficiency has been reduced. Significantly greater idle than average
  • Thermometer with a very low temperature range

Keep in mind that all of the symptoms listed might be attributed to a variety of different reasons. As a result, it is not possible to state categorically that when the temperature gauge is consistently low, the problem is caused by a P0128 coat. Using an OBDII scanner, you may validate and obtain reliable findings by connecting it to the engine and scanning for any fault codes that may exist. What is the solution to the P0128 error code? Because the P0128 error code may be addressed at home without the assistance of a professional technician, you can save hundreds of dollars on labor expenses.

Use an OBDII scanner to confirm the error

It is important that we are all on the same page before listing the many DIYs that we have inspected. This may be done by first validating the mistake and then completing the entity otherwise. It is possible that many individuals may believe that the problem is tied to AP0128, but this is not the proper way to proceed. Consequently, connecting the OBDII scanner to your vehicle’s computer and scanning for problems will take you no more than a few minutes at most. The problem codes will be shown on the screen by your OBDII scanner when they have been scanned and defined by the scanner.

Be aware that certain inactive or old errors may appear on the screen as well; as a result, double-check that you are looking at the most recent or active error codes before attempting to correct them.

Inspect the ECT sensor

A defective ECT sensor is one of the most prevalent faults that might cause a trigger to occur. The sensor, which is normally attached to the thermostat, should be located. It is always possible to refer to your vehicle’s owner’s handbook for more detailed and precise instructions if you are having difficulty locating the ECT sensor. Locate the sensor and examine it closely for any indications of damage or degradation when you’ve located it. Check the sensor’s performance with a multimeter that is linked to it to see if it is performing at the required level.

If this is not the case, then the ECT sensor will need to be replaced immediately.

Check the thermostat

Assuming that the ECT sensor is functioning properly, the next step is to examine the thermostat itself in further detail. Locate your thermostat by consulting your vehicle’s owner’s handbook, then gently remove it from the vehicle. Keep in mind that you should avoid touching the engine while it is really hot since the thermostat may get quite hot, and messing with the coolant can be highly dangerous. Consequently, once the engine has cooled down, you may proceed with any examination or thermostat replacement that is necessary.

As a result, you will need to replace the thermostat and ensure that it is functionally comparable to the previous one before attempting to install it.

In order for the thermostat to work, it must be linked to the engine on one side and to the hose on the other.

A helpful advice in this case is to snap a picture of the old thermostat and use it to guide you through the process of installing the new one. Alternatively, you may always have the vehicle’s owner’s handbook on hand for more extensive instructions.

Clear the error code

Once the problem has been resolved, you must make sure that the error code has been manually cleared. Any internal problem codes linked with the check engine light will not be erased on their own by the check engine light. You must scan the vehicle with an OBDII scanner and request that any inactive codes be cleared. Once you’ve cleared the codes, you’ll be able to check that the problem has been resolved. In addition, it is critical that you do a second scan to guarantee that the area code does not surface a second time.

  1. When you take your automobile to a repair shop, they may examine it more closely and determine if you completed the DIY correctly or incorrectly.
  2. Is it legal for me to drive my automobile with a P0128 code?
  3. It is important to note that the only time you should stop driving this car is if the coolant level is low.
  4. What is the approximate cost of resolving the P0128 error code?
  5. As an example, if the problematic component that caused a P0128 code was a bad coolant temperature sensor, repair charges ranged from $140.00 to $190.
  6. Keep in mind that the cost of labor is a significant component of any repair expenses associated with the P0128 error code.
  7. This is because dealerships charge a high hourly rate.

While repairing the vehicle, there is a very little probability of introducing new problems.

P0128 codes are generally inexpensive to solve, therefore repairing them now to avoid severe problems down the line is always a good investment.

Example: If you know that your automobile has serious transmission or engine problems, it is never worth spending a few of hundred dollars to correct a P0128 code on your dashboard.

Is it possible for a thermostat that has become jammed open to cause a P0128 code?

Did you know that when a thermostat malfunctions, it is programmed to force the door to open automatically?

As a result, a thermostat that is jammed open is a fairly common source of the P0128 error code.

However, your engine must achieve its optimal operating temperature in order to function effectively.

Despite the fact that the P0128 code is not as dangerous as some others, it is critical that you as a driver take care of it as soon as you see it in order to avoid dealing with major difficulties down the road in the future.

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P0128 Code – What Does It Mean & How To Fix It

This is one of the most often encountered OBD2 error codes. Read the rest of this article to find out what it means, how to solve it, and what additional codes may appear that are associated with it.


The presence of a P0128 fault code indicates that your coolant thermostat is experiencing a malfunction. The problem is that the coolant temperature is lower than the temperature at which the thermostat is set to regulate it.


In its most basic form, the P0128 fault code indicates that the engine coolant is not becoming heated enough, or rapidly enough. It comes down to the fact that the engine does not achieve its operational temperature within a particular length of time, which indicates a problem with the engine. The engine control module (ECM) is responsible for determining and regulating the length of time that this process takes, which is determined by the temperature within the vehicle.


Several typical problems can result in the P0128 trouble code, including the following: One of the most typical causes of this problem is that the engine coolant thermostat has become stuck open or is functioning at an excessively early temperature. The second most likely reason for this is that the real coolant temperature sensor or the cable that is linked to the sensor is malfunctioning or has been damaged. It is possible that both or one of these problems will result in the P0128 error code.


If you have a P0128 issue code on your car, you will most likely experience the following two symptoms:

  • This is due to the low coolant temperature in the engine, which causes the heater to not heat up sufficiently. If your automobile takes an excessive amount of time to warm up, or if you notice that the temperature gauge does not rise as high as it used to, you may have a problem.

Observing or operating a vehicle while experiencing one of the symptoms listed above may be an indicator or validation of a P0128 issue code.


A mechanic will go through a number of logical procedures in order to correctly diagnose and validate a P0128 trouble code in the vehicle. Keep in mind that diagnostic procedures and measures might differ from one mechanic to the next, but in general, there will be a set of actions that each mechanic should take in order to analyze this particular difficulty code:

  • Checking the coolant temperature as the coolant starts running through the radiator hose and into the thermostat should be your first order of business, after that. In general, the radiator hose should be barely warm until the thermostat is activated
  • After the thermostat is activated, coolant that is extremely hot should begin to flow through the radiator hose, rapidly warming it. You must exercise caution since the temperature is high enough that you might be burnt
  • If you see that the radiator hose steadily warms up and does not immediately or suddenly become hot, this is an indication that the thermostat is locked in the open position. Assuming that the hose heats up rapidly as it should, the next item you should verify is the reading you receive from the coolant temperature sensor. To ensure that your reading is correct, use a scan tool, but you may also use your car’s temperature gauge to confirm the data. Make a comparison between the reading you acquire and the reading obtained by an infrared thermometer. If everything is operating properly, the thermometer should generally read about 200 degrees Fahrenheit. If not, the temperature should be higher. Unless the scan tool reading matches (or is very near to matching) the thermometer reading, there is a problem with the sensor inside the automobile
  • If the sensor is the problem, the connections will normally degrade as a result of the sensor failure. A faulty connection may be the root of this issue, which may in turn lead to more problems with the car. Even though the connector appears to be in working order, the sensor might be malfunctioning
  • In rare instances, an issue with the supply voltage supplied by the ECM could cause a P0128 trouble code to be shown on the dashboard. Detailing this topic is challenging since you will want factory-level information that is particular to your vehicle and changes from vehicle to vehicle. If you want to learn more about this, check out the links below. If you want to avoid inflicting harm to the car, you’ll need to be familiar with electrical diagnostics.

Common mistakes

The most typical error people make when dealing with a P0128 problem code is to replace the thermostat without first thoroughly inspecting the rest of the vehicle’s components.

How serious is this?

Although this is not a very dangerous difficulty code, it should be noted that if the vehicle is used in this manner for an extended length of time, it may result in more serious problems. A tiny probability exists that water contamination of the oil will cause degradation and engine damage in the event of an accident. As a result of the engine coolant not becoming hot enough, this code is generated, which indicates that the engine is not getting hot enough to burn off the condensation that has built up in the engine.

Even while this particular issue of condensation may not manifest itself in visible harm for several years, the possibility of ruining your engine remains.

What repairs can fix the code?

The simplest fixes that you may make in order to resolve the error code are as follows:

  • Replace the thermostat with a new one. This is by far the most prevalent type of repair. It may be necessary to replace the coolant temperature sensor, as well as the cabling and/or connection that are related with the sensor that has been damaged.

Related codes

There are none listed.


Currently, there aren’t any.

What is Trouble Code P0128 – Coolant Below Thermostat Regulating Temperature

Thermostat for the coolant (Coolant Temperature Below Thermostat Regulating Temperature)

Here is what a P0128 means, in simple terms

The P0128 trouble code indicates that your engine is operating at an excessively high temperature. This sounds fantastic, doesn’t it? In actuality, it isn’t. An engine that is unable to reach the necessary temperature in the requisite amount of time may result in an increase in dangerous emissions. So the Engine Control Module (ECM) is configured to recognize low engine coolant temperature (when compared to ambient air temperature and a pre-determined value), as well the pace at which the pre-determined engine coolant temperature must be achieved.

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What caused my vehicle to set a P0128 trouble code?

The Engine Coolant Temperature (ECT) sensor is used by the engine control module to assess whether or not an OBD-II P0128 code should be generated. Any time a trouble code P0128 appears by itself, a shop technician is likely to believe that the sensor for the engine coolant temperature or the thermostat is malfunctioning.

  • A coolant thermostat that is either jammed open or opens prematurely
  • Engine coolant temperature (ECT) sensor and/or ECT wire (most commonly at the ECT connection/harness) that are not functioning properly

What symptoms will my vehicle experience when trouble code P0128?

  • Your vehicle’s check engine light (CEL), Malfunction Indicator Lamp (MIL), or Service Engine Light (SEL) is illuminated when the engine is running. When the ECC detects an issue with the engine coolant temperature, a light will be lighted and freeze frame data will be saved, containing information on the engine sensor data at the moment of detection. In addition, the trouble code P0128 will be logged in the ECM ROM memory. The engine will take an excessive amount of time to warm up (particularly on cold days)
  • Because it uses the engine’s coolant to heat the vehicle’s interior, the air conditioning heater may not be able to achieve the desired temperature because the coolant never reaches its maximum working temperature. Because the correct engine operating temperature is not attained quickly enough, the smell of HC (hydrocarbon) lingers for a longer period of time than is typical during engine beginning.

How can I fix a P0128 problem and where should I start?

We recommend that you start by checking to see if your engine’s thermostat is functioning properly. A properly functioning thermostat should only let the flow of coolant after the coolant has reached the right operating temperature (usually around 200f). Start your engine (from a cold start) and observe the temperature of the radiator hose that is connected to the thermostat housing to determine whether or not the thermostat is working properly. To begin with, the hose should be chilly to the touch, but it should quickly become quite hot (after around 3-5 minutes depending on ambient air temperature).

  • If you do not notice a sudden increase in the hose’s exterior temperature and instead feel the hose gradually getting warmer, your vehicle may have a thermostat that is stuck open, which allows coolant to flow through the radiator immediately after the engine is started.
  • Providing the thermostat is performing as it should, the next component to be checked is the engine coolant temperature sensing device.
  • Check for reference voltage from the ECM to the ECT while the ignition is turned on and the engine is turned off.
  • After that, start the engine and measure the voltage across the ECT signal line.
  • However, because the values vary from car to vehicle, we will not go into specifics about what voltage you should be seeing here.
  • The most crucial thing to pay attention to is whether or not there is a voltage fluctuation.

As a final precaution, make certain that you take the reading after the ECT connection and at the ECT sensor. In this way, you may establish whether the problem is caused by a faulty connection, which might save you money by not having to replace the ECT needlessly.

  • The coolant hose attached to the thermostat housing should be checked periodically for a sudden increase in temperature. Examine the ECT signal and the reference voltage for any anomalies. Check the ECT connection for damage.

Can I drive my vehicle with a P0128 trouble code and illuminated CEL?

  • You may be able to use your car while the P0128 problem code is displayed. Except for the possibility that the temperature gauge never reaches normal levels and that the Check Engine light is lighted, you will not notice any problems with your vehicle. In addition, if your vehicle is required to undergo smog testing, the presence of the P0128 trouble code and the presence of the check engine light will result in the vehicle failing the test. The check engine light or malfunction indicator lamp is illuminated while the P0128 trouble code is recorded in the ECC.

Preventive Cooling System Service: Keeping P0128 Codes Away

Given the current spectrum of extended maintenance intervals, it’s reasonable to assume that your shop will only encounter Grandma’s 2011 Volkswagen Jetta when the oil life monitor indicates that an oil change is required. At this point, the most pressing concern is that Grandma may have driven her VW exclusively to church on Sundays for several years without ever opening the hood of the vehicle. More information is available by clicking here. The cooling system, which is the more important of the engine’s two critical fluid systems, lubrication and cooling, may require more frequent maintenance.

With that in mind, let’s take a look at a few different methods of inspecting cooling systems on cars with longer maintenance intervals.


Photo 1: Inspection of the clear plastic coolant reservoir situated in the engine compartment is the first step in cooling system repair (see Photo 1). It is possible that the ordinary reservoir will require a cup or two of make-up coolant after 10,000 miles of driving to compensate for natural evaporation loss. In contrast, if the coolant level is significantly lower than the ‘add’ line, the car is losing coolant either from an exterior leak or through an interior leak in the cylinder head gasket or the intake manifold.

  1. 2 It’s always a good idea to check for leaks in a cooling system by applying pressure to it when the engine is cold (see Photo 2).
  2. Always check the freezing point of the coolant and refer to service literature and technical service bulletins (TSBs) for information on coolant replacement intervals and cooling system performance concerns.
  3. As the coolant volume expands and contracts in response to variations in engine temperature, coolant flows around the pressure relief valve on the cap and through the coolant inlet valve on the cap’s pressure relief valve.
  4. For example, one shop misinterpreted a perplexing no-code stalling issue as a broken crankshaft position sensor after receiving a call from the customer.
  5. The stalling problem arose as a result of the ECT sensor no longer contacting the engine cooling system.
  6. A second example is a complaint of overheating that is caused by a clogged return line to the coolant reservoir, which is identical to the first.
  7. Photo No.
  8. The gel formation depicted in the photo below was generated either by the mixing of various coolants or by the addition of non-compatible additives, which brings me to a brief discussion on the topic of combining coolants.
  9. In accordance with more thorough aftermarket service instructions, some Volkswagen vehicles from 1998 to 2005 use a non-silicon-based coolant formulation.

When a non-silicon-based coolant is mixed with a silicon-based coolant, gelling and subsequent blockage of internal coolant passageways occur, which all contribute to driveability issues caused by clogged coolant reservoir return lines.


The fact that today’s onboard diagnostics are so thorough makes it unnecessary for me to connect an improved or professional-level scan tool in order to check the status of the cooling system electronics. Before we get started, let’s take a look at how OBD II diagnostic trouble codes (DTCs) are processed and stored in the vehicle’s diagnostic memory. They are ‘global’ or ‘generic’ designations that have been allocated by the Environmental Protection Agency to a particular type of pollution (EPA).

It is also possible for a worldwide code to be shown as a ‘pending’ DTC, which signifies that the failure must occur on two different instances before the orange malfunction indicator light (MIL) would activate.


Diagnostic issue codes can offer a detailed history of the cooling system’s functioning in the past. The extremely common P0128 DTC, which indicates inadequate coolant temperature, may be present in any of the three statuses: current, pending, or history, depending on the circumstances. According to an aftermarket source, the enabling conditions for a P0128 on a 2010 Hyundai Accent with a 1.6L DOHC engine include the following elements: The temperature of the surrounding environment must be between 18.5o F and 113o F.

  • There is more than 0.4 lb of integrated air mass in this system.
  • The temperature of the cold-start coolant must fluctuate between 18.5o F and 113o F.
  • Low-speed driving must be at least 25 miles per hour.
  • When the collected air mass exceeds a certain threshold, the coolant temperature must be less than 168.4o F.
  • As a result, the first P0128 failure may be located under ‘pending codes’ in the Hyundai’s diagnostic trouble code database.
  • If the coolant level has subsequently been replenished, the P0128 may no longer be active and, as a result, may be identified in the history of diagnostic trouble codes.
  • It’s important to note that if the car has been cold-soaked overnight, the ambient and coolant temperatures should be within a few degrees of each other on the scan tool data.
  • In sharp contrast, the 2011 Volkswagen Jetta owned by granny might have as many as 15 probable DTC cooling system issues listed in the same aftermarket source as the car in question.

Most nameplates feature a bidirectional electric cooling fan scan tool test, which is common in the industry. To make sure that all fan relays and hardware are functioning as intended, run the fans at all three speeds: low, moderate, and high.


Coolant loss, loud bearings, and overheating problems are all signs of a failing water pump, according to the manufacturer. Remember that, while most water pumps are belt-driven, computer-controlled electric water pumps are now being utilized on certain import nameplates, so keep that in mind. A water pump can fail for a variety of reasons, including bearing noise, seal failure, or impeller failure, among others. It is typically possible to monitor water pump pressure by splicing a pressure gauge into the heater-hose output when there is a suspicion that a coolant circulation problem exists.

While the results may vary depending on the application, the pump should be able to generate 15-20 psi at the heater hose output while running at around 2,500 rpm.


Photo No. 4 Unlike traditional serpentine belts, EPDM rubber serpentine belts do not develop the cracking and weathering markings that conventional belts do. Instead of inspecting the belt grooves for cracks, it is preferable to use a wear gauge to assess wear in the grooves (see Photo 4). EPDM coolant hoses, on the other hand, show no evidence of weathering or wear. If you have an engine repair or a radiator replacement, a fair rule of thumb is to replace, rather than reinstall, any contemporary cooling system hoses that have been installed recently.


It is critical to conduct customer interviews, especially when identifying radiator problems. On hot days, overheating complaints when ascending long slopes or idling in heavy traffic are typical signs of poor radiator functioning. Air must be allowed to easily move through the radiator core since the radiator’s duty is to disperse heat away from the engine and into the surrounding environment. However, while paneling must frequently be removed in order to inspect both the air conditioner condenser and the radiator core for external clogging caused by road debris, a thorough cleaning of the external radiator core with low-pressure water and compressed air is a good preventive maintenance procedure to perform.

How Long Can You Drive With A P0128 Code Mercedes Benz

It is always a source of anxiety when error codes appear on a car, but this is especially true for Mercedes-Benz automobiles. The German company is renowned for providing unrivaled comfort and technology, but it is also notorious for charging extremely hefty maintenance and repair fees. As a result, it is totally normal that you might want to delay taking action, especially if the problem is minor. The P0128 error number indicates that there is a problem with the thermostat. If the coolant temperature gauge remains within a few degrees of the recommended operating temperature, you can continue to drive with the P0128 code for an unlimited period of time.

It is necessary to replace the thermostat if the engine is operating at a low temperature. Fortunately, this is a pretty straightforward process, and you’ll discover exactly how to complete it in this guide!

What Does the Mercedes P0128 Code Mean?

‘Coolant temperature below thermostat controlling temperature,’ according to the P0128 error code description. The water pump is responsible for forcing coolant to flow around the engine area. When it reaches the engine, it absorbs the heat generated by the engine and continues forward at a higher temperature than before. The thermostat sensor and valve are the next stop on the route. Meanwhile, the sensor collects data for use by the car’s computer and driver feedback, and the thermostat valve automatically controls the flow of coolant through the vehicle.

As the Mercedesovercomes air resistance, coolant flows from the thermostat to the radiator, where it is cooled by the tremendous volume of air hitting it as the Mercedesovercomes air resistance.

What Are The Causes?

Generally speaking, a faulty thermostat valve is the most typical reason of a P0128 code. Make sure to read the article ‘Why does my car’s temperature gauge move up and down?’ for more information on additional symptoms of thermostat valve failure, since there are many more signals to be aware of. However, because the coolant temperature sensor is significantly less likely to fail than the thermostat valve, the conventional course of action is to replace the thermostat valve first, and if the error code does not go away, then replace the coolant temperature sensor.

See also:  Hyundai clicking sound when turning? (TOP 5 Tips)

How Long Can You Drive With a P0128 Code?

I am unable to provide you with an actual estimate since the situation is not as straightforward as it appears. As an alternative, I’ll provide you with an understanding of just how terrible the P0128 is from a different standpoint. What is it that all athletes have in common, regardless of their sport or ability level? They begin by warming up. To avoid muscle and joint pain or damage, it is vital to warm up before doing any activity. In some aspects, the engine is no different than the rest of the vehicle.

In order for the system to function properly, it must maintain a ‘optimal operating temperature,’ which is typically between 180 and 220 degrees Fahrenheit.

Additionally, the cold engine ‘runs rich’ in addition to having insufficient lubrication.

Given that this will never occur, the injectors will continue to overflow, resulting in a greater accumulation of deposits, a loss of power, and a greater consumption of fuel.

According to the manufacturer’s specifications, you could technically drive your Mercedes for hundreds, if not thousands of kilometers without having any problems. You shouldn’t put off the repair indefinitely, though, because the engine is progressively degrading beneath the hood of your vehicle.

How Do I Repair the P0128 Code?

Fixing a Mercedes-Benz at home is often regarded to be an impossible task, however repairing the P0128 error code should be a straightforward DIY project. Some individuals would advise you to wait until the problem has been adequately diagnosed before replacing any parts. This is only partially accurate; in certain cases, issues can only be detected by ruling out a number of possibilities at a time. Given that neither the valve nor the sensor can be seen inspected, our only alternative is to replace them with new ones.

By following this reasoning whenever possible, you’ll end up with a larger bank account as well as an engine with far more healthy components.

Replace the Thermostat

You’ll need the following equipment and supplies to complete the thermostat replacement:

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Top X AUTOHAUX Car Auto Thermostat Housing 2722000515 for Mercedes-Bens CL550 E550 G550
Top Lumax LX-1628 Black 6 Quart Plastic Drain Pan. to Collect The Oil in Oil Changes. The Rugged, Oil Resistant All-Purpose Plastic Pan Will not Rust or Dent. Easy Cleaning.
Top TEKTON Stubby 1/4 Inch Slotted High-Torque Screwdriver (Black Oxide Blade) | 26621
Top Wekster Wide Mouth Fuel Funnel with Handle – Large Plastic Automotive Funnels, Long Flexible Spout Extension, Removable Mesh Screen Filter for Water, Gasoline, Coolant, Transmission, Engine Oil
Top CARBYNE Master Torx Bit Socket SetTorx External Socket Set – 62 Piece, S2 Steel Bits, CrV Sockets | 1/4-inch, 3/8-inch1/2-inch Drive
Top Pro-LifT F-2315PE Grey Hydraulic Trolley Jack Car Lift with Blow Molded Case-3000 LBS Capacity
Top BIG RED T43202 Torin Steel Jack Stands: 3 Ton (6,000 lb) Capacity, Red, 1 Pair

Prices were obtained via the Amazon Product Advertising API on the following day: The car jack and stands are there to make it easier for you to get to the coolant drainage plug when you need to. If you believe you are capable of performing the work without their assistance, go ahead! The oil pan is a convenient tool to have on hand, but you may substitute any clean bucket you have lying around the garage. As a rule, Mercedes use E Torx sockets, so make sure you have a suitable socket set on hand before starting.

Steps to replace the thermostat:

  1. Lifting the vehicle or immediately accessing the underbelly are both options. Some skid plate designs have a tiny cutout cover that may be removed to provide direct access to the drainage hole. Others do not. It may be necessary to remove it using a socket wrench if yours does not have this feature. It is possible to drain the coolant through a drain plug placed beneath the radiator. Place the bucket or pan beneath and loosen the plug with a flathead screwdriver
  2. Then repeat the process. Open the coolant cap on the top of the vehicle to allow air to enter and speed up the cooling process
  3. After locating and removing the thermostat housing, use the flathead screwdriver to loosen the hose clamp and pull it away from the thermostat. Continue to maintain the container below the car in case more coolant leaks out. Torx sockets in the reversed E position are used to unscrew the thermostat housing. If it is stubbornly refusing to come off, tap the housing with the rubber end of your screwdriver until it comes loose. Remove the thermostat, and don’t forget to remove the rubber gasket as well. Ensure that the contact surface between the new thermostat and the engine is well cleaned. Insert the replacement rubber gasket and the thermostat into the housing
  4. The radiator hose should be reattached once the housing screws have been tightened. Close the drainage plug and reinstall the skid plate as necessary. Return the old coolant to the system using the funnel and strainer, or replace it with a new batch if you like

A Mercedes-thermostat Benz’s is simple to replace, and there isn’t much that can go wrong if you follow the instructions and work properly. The only problem you could have is that you won’t be able to refill the cooling system with the same amount of coolant as before. This implies that there is an air pocket in the system, which we will have to clear out before we can continue. After leaving the coolant cap open, carefully follow the radiator hoses around the engine and squeeze them slowly in the places where they curve.

You’ll hear a bubbling sound when you squeeze the air pocket you’ve discovered.

Even if you haven’t had any luck, you might try leaving the coolant bottle open and starting the engine for a few seconds.

If it doesn’t work the first time, go through the entire process again.

Replace the Coolant Temperature Sensor

You’ll need the following tools to complete the coolant temperature sensor replacement: Prices were obtained via the Amazon Product Advertising API on the following day: Using almost similar equipment and procedures, you may replace the coolant temperature sensor in the same manner that you did while changing the thermostat housing. Because the sensor may be attached by hand, you won’t need to use the socket set. If you’re quick enough, you might even be able to avoid emptying the coolant altogether — simply pull the sensor out and replace it as quickly as possible with the new one.

It’s preferable to drain at least some of the coolant so that the level does not rise to the point where it can damage anything, rather than risk damaging something.


The P0128 error code appears when coolant temperatures are within the maximum allowable range, rather than above it. It is possible that a blocked radiator might cause coolant temperatures to rise, perhaps resulting in an overheated engine. If you are experiencing any of these problems, please see our tutorial on cleaning the radiatorhere.

How do I fix code P0128?

It is recommended that you replace the thermostat valve first, and if it does not help, you should then replace the thermostat sensor. If neither of these factors are contributing to the problem, the ECU may be to blame; thus, you should seek expert assistance at this point.

Can I drive my car with a P0128 code?

A P0128 code will not prevent you from driving in any manner, but driving with low coolant temps for an extended period of time might cause lasting engine damage.

How is P0128 diagnosed?

In all honesty, issues such as the P0128 error code are resolved first and then diagnosed. It’s normally the same price as a couple of spark plugs to replace a thermostat valve, so if you suspect yours has failed, it’s more time-efficient to simply replace it rather than attempting to identify the problem.

Can a stuck thermostat fix itself?

Even while it is not unheard of for a frozen thermostat to reactivate, it is important to remember that you are operating on borrowed time. No need to rush into getting it fixed, but make an effort to find some spare time and replace it before it becomes stuck again.


While it is possible to continue driving while a P0128 code is flashing on your dashboard, leaving it unattended might result in catastrophic engine problems. Most likely, the thermostat valve and coolant temperature sensor are to blame. Both are inexpensive and simple to repair, so instead of letting your mechanic charge you upwards of $250 in labor, get the components and fix them yourself!

P0128 Code: Low Thermostat Temperature (Symptoms, Causes, and Fixes)

The most recent update was made on October 14, 2021. The P0128 error code indicates that your car’s coolant temperature reached an unusually high level after you had driven it for some time. Are you looking for a reliable online repair manual? The top five choices may be found by clicking here. Within 15 minutes of running the engine, the temperature of the engine coolant should be about 160° F (71° C). The powertrain control module (PCM) analyzes signals from the coolant thermostat to assess whether or not it has reached the desired temperature within the specified timeframe.

What Does Code P0128 Mean?

P0128 is an OBD-II trouble code that has a description. The temperature of the coolant thermostat is lower than the regulating temperature. After you start your car, the coolant remains within the engine block to absorb heat until it reaches the proper temperature, at which point it is released to flow throughout the vehicle. The fuel-to-air ratio is greater during this period, allowing the engine to burn more gasoline while warming up. Once the required temperature has been attained, the PCM transfers control of the fuel ratio to the oxygen sensors, resulting in improved fuel efficiency and reduced emissions.

One of the sensors it employs to do this is your coolant thermostat.

Error code P0128 is generated when the engine coolant in your car does not heat up quickly enough, or when the signal returned by the coolant thermostat is either missing or inconsistent, as previously indicated. Code P0118 is another example.

Symptoms of Code P0128

In other cases, the sole symptom of code P0128 is a check engine light, which is what prompted you to utilize your code reader in the first place. However, in rare instances, it can also result in the following:

  • Temperatures in the engine that are irregular or fluctuate as the vehicle is being driven
  • It is possible that the automobile will take an exceptionally lengthy time to heat up, especially in colder weather. The performance of an air conditioner may be better than usual, while the performance of a heater may be low. Consumption of excessive amounts of gasoline Idle time is really long.

Causes of Code P0128

Anything that causes uneven coolant temperatures or an interruption of the signal from the coolant thermostat to the PCM has the potential to cause code P0128 to be triggered. The following are the most typical causes:

  • The most typical reason of code P0128 is an aleaky or jammed engine coolant thermostat
  • However, there are other possible causes. The P0128 code can also be caused by faulty engine coolant temperature sensors, either due to faulty wiring or faulty sensors. Radiator Fan Failure– The radiator fan in your automobile is responsible for cooling the engine and the coolant. If it remains in the on position, the coolant will heat slowly as a result of the resistance. Low or dirty coolant– Temperature levels will be affected by poor coolant system conditions. The majority of the time, this is due to an insufficient amount of coolant, although occasionally unclean coolant is to fault, which can cause the thermostat to stick
  • Nonetheless, Lastly, the PCM itself may be failing and incorrectly emitting error signals
  • This is known as PCM failure.

Is Code P0128 Serious?

The error number P0128 is not critical enough to necessitate urgent repair or to prohibit you from using your car safely. However, because it has the potential to have a significant influence on fuel efficiency and emissions, the problem should be addressed as soon as possible. Prolonged coolant temperature issues can potentially cause catastrophic engine damage if left unattended. The reason for the appearance of the code has an impact on how rapidly the damage might manifest itself. In summary, while you don’t have to pull over or go to the shop right away, you do want to get a diagnosis on the problem as soon as possible.

How to Fix Error Code P0128

In order to diagnose error code P0128, the first step is to determine whether or not the engine coolant thermostat is jammed open or leaking, as this is the most typical cause of the code. Check to see that the car is entirely cool before starting it and placing your hand on the radiator pipe to cool it. Keep an eye on the hose because, if everything is functioning properly, it will get quite hot. If it warms up slowly or not at all, it is probable that the thermostat is stuck or leaking and will need to be cleaned or completely replaced.

A thermostat that is clogged due to dirty coolant might be dangerous.

You should also listen to check whether the radiator fan is stuck on or if a section of the radiator line has collapsed.

Contrast the reading provided by your scan instrument with a manual reading taken using an infrared thermometer.

Ensure that the readings are consistent by inspecting the sensor’s wiring and connections, and replacing or cleaning them if required.

If the code returns, it is possible that the sensor will need to be replaced.

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