Jeep P0128? (TOP 5 Tips)

  • P0128 is a relatively common OBDII trouble code. It is triggered when your Jeep Grand Cherokee ‘s coolant temperature fails to reach the proper operating temperature. It’s a generic powertrain code, which means that regardless of who manufactured the vehicle, it’ll have the same meaning (1996+).

Is it safe to drive with P0128 code?

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.

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 causes P0128 code?

The most common cause for P0128 is the engine coolant thermostat is stuck open. When the thermostat does open, the hot coolant should start to flow and quickly warm up the radiator hose. If the radiator hose heats up slowly, the thermostat is stuck open or opening prematurely and needs to be replaced.

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).

What happens if engine runs too cold?

If you run your engine cold all the time, you’ll most likely experience increased fuel consumption across the board. Additionally, you’ll find higher amounts of carbon buildup at various parts of the engine. The most damaging effect of feeding a fuel-rich mixture to the engine is excess fuel reaching the exhaust.

How do I know if my coolant temp sensor is bad?

Bad Coolant Temperature Sensor Symptoms

  1. Check Engine Light.
  2. Poor Mileage.
  3. Electrical Cooling Fans not coming on.
  4. Black Smoke from the Exhaust Pipe.
  5. Difficult Starting Condition.
  6. Engine Overheats.
  7. Poor Idling.
  8. Poor Engine performance.

Will a bad thermostat throw a code?

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.

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.

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.

How do you fix low coolant temp?

The easiest way how to fix low coolant is to begin filling your coolant back to acceptable levels and allow the coolant to soak back into the system. Upon opening your radiator cap, you will see indicators such as “max” and “min” which indicates the level of the coolant mixture currently in your tank.

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 causes a P0172 code?

Here are the different possible reasons why you’re getting the engine code P0172: Contaminated engine oil (too long since the last oil change) A leaking fuel injector. Excessive fuel pressure due to restriction along the fuel return line or a faulty fuel pressure regulator.

Jeep P0128 – Meaning, Causes, Symptoms, & Fixes

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

JeepCode P0128 Meaning

In your Jeep, the powertrain control module (PCM) keeps track of how long it takes the engine to achieve and maintain the proper operating temperature. As soon as the proper engine operating temperature is attained, the powertrain control module instructs the fuel system to enter ‘closed loop,’ in which the readings from the oxygen sensors are utilized to maintain the efficient air to fuel mixture, which is 14.7:1 by default. The majority of PCMs require that the engine coolant temperature sensor record temperatures greater than 160 degrees Fahrenheit within 15 minutes of the engine being started.

Your Jeep’s PCM will make a note if any of these two requirements is not satisfied; otherwise, nothing will be recorded.

Jeep P0128 Symptoms

  • Make sure the engine is working properly. The light is on
  • The idle speed is higher than usual
  • Fuel efficiency has been reduced. The temperature gauge has dropped to an exceptionally low level.

Jeep P0128 Causes

(The asterisk denotes the most common)

  • Faulty coolant temperature sensor
  • Faulty wiring for the coolant temperature circuit
  • Radiator fan that is continually operating
  • Stuck open thermostat*

Jeep Code P0128 Severity – Low

A thermostat that has become stuck open is most likely the source of the problem; nevertheless, thermostats are often constructed such that when they fail, they remain open, enabling full flow of coolant to safeguard your engine. Check your coolant level first to ensure that it is fully charged before continuing to drive the Jeep’s engine. In the event that your engine overheats, it will result in engine failure.

Jeep Code P0128 Common Diagnosis Mistakes

Many people just replace the thermostat in their Jeep without investigating all of the potential issues. Examine the cooling system for rust deposits or a blend of different coolants to determine its condition. When changing the thermostat, make sure to clean the engine block and radiator thoroughly to avoid deposits from becoming lodged in the new thermostat housing.

Jeep Code P0128 Diagnosis Steps

  1. Check to see if P0128 is the only code present on your Jeep by scanning it. If there are any additional codes present, they must be dealt with first. Check the level and condition of the coolant. If there is an excessive amount of rust and poor coolant quality, this can cause the cooling system to clog up and the thermostat to become unresponsive. 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 in response to temperature changes, the coolant temperature sensor in your Jeep should be replaced, or the sensor’s wiring should be repaired if it is broken. The most typical reason for P0128 in a Jeep is that the engine coolant thermostat has been stuck in the closed position. This may be easily diagnosed by feeling the radiator pipe and monitoring how hot the coolant becomes as it begins to flow through the radiator hose
  2. However, you must exercise extreme caution when doing so since you could be burnt. The hose should only be somewhat heated until the thermostat is activated. The heated coolant should begin to flow through the radiator line as soon as the thermostat is activated, immediately warming it. 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 the gaskets and coolant. Test drive the vehicle and keep an eye on the coolant temperature. Look for any leaks.

Jeep Wrangler P0128: Coolant Thermostat – Below Temp

It is a rather common OBDII issue code, with the code P0128 being the most prevalent. It is activated if the coolant temperature in your Jeep Wrangler does not reach the correct operating temperature within a specified time period. It is a generic powertrain code, which implies that it will have the same meaning regardless of who made the car (1996 and after).

Wrangler P0128 Symptoms

Unless it becomes chilly, there aren’t many symptoms associated with P0128. The following are the symptoms that are related with this error code:

  • It will be difficult to maintain a comfortable temperature inside your Jeep Wrangler if you have the P0128 problem code on your dash. When the thermostat fails to get the engine up to the right operating temperature completely, it indicates that the water passing through the heater core is not as hot as it should be. As a result, there is reduced or no heat. With the exception of the heater not functioning properly, the only additional symptom associated with P0128 is often the presence of the service engine soon light itself

P0128 Causes: Jeep Wrangler

Due to the fact that the thermostat is responsible for 90 percent of all P0128 errors, repairing the code is typically a straightforward process. The following are the most often seen causes of P0128:

  • Generally speaking, the most common reason of the P0128 error code is going to be a thermostat that has been left open for an extended period of time. When a thermostat is jammed open, coolant is allowed to run through the engine at all times, which is dangerous. When this occurs, the engine is said to be ‘overcooled,’ and it never achieves the correct operational temperature. How to Replace a Thermostat (with Pictures) (video). The Wrangler Thermostat is a device that regulates the temperature of a building. A issue with the wiring/sensor related with engine water temperature sensing or with the coolant temperature sensor itself may occur if the thermostat is not malfunctioning.

Jeep Wrangler P0128 Diagnosis

Checking if the engine is running at its proper operating temperature will help determine whether or not the problem is due to a sensor malfunction or a problem with the thermostat. When driving in a car that does not have a temperature gauge, this might be extremely difficult to determine. A decent approach to at the very least get a good indication of whether or not the problem is with the thermostat or the sensor is to start the car cold and let it idle for ten or so minutes before diagnosing the problem.

In addition, a mechanic will often inspect the area under the hood where the radiator hose meets the thermostat housing for problems.

If it is simple to squeeze the hose, it is likely that water will be running through it at the time. Ensure that you are conscious of the belting attachments as well as the fan in the event that you pinch your hose. The fan may be turned on and off at any moment.


In the short term, P0128 does not pose a severe threat to the driveability of your Jeep Wrangler. It should be mentioned that most contemporary fuel-injected engines prefer to operate at temperatures of 200 degrees or higher. Allowing them to run at too low a temperature for a lengthy period of time might cause difficulties. Good luck figuring out what’s wrong with your Jeep Wrangler!

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Code P0128 – Coolant Temperature Below Thermostat Regulating Temperature

Date last updated: July 31, 2021 The codeP0128 – Coolant Temperature Below Thermostat Regulating Temperatureindicates that the engine has not reached the anticipated temperature after being operated for an appropriate amount of time, or, in other words, that it is taking an excessive amount of time to warm up. In frigid conditions, this code frequently causes the Check Engine light to illuminate and then turn off.


There might be no symptoms at all, or there could be any of the following symptoms associated with the code P0128: Because of the cold weather, the engine takes a long time to warm up; the engine temperature lowers when driving on the highway, especially in the winter; the air conditioner and (or) temperature gauge quit working; and (or) the Check Engine light illuminates (GM).

Is it safe to drive with the code P0128?

If the car is in good running condition, the coolant level is enough, there is sufficient heat, and there are no other driving difficulties, it is conceivable that the thermostat is stuck intermittently. In general, this is not a major issue, however it may have an impact on the gas mileage in some cases. We would nevertheless recommend that you get the car inspected by a mechanic.


Low amount of coolant Sensor for measuring the temperature of the coolant The following are examples of common causes: Incorrect or stickythermostat – The thermostat seal is not sealing properly, allowing coolant to bypass a closed thermostat – Faulty engine coolant temperature (ECT) sensor – Engine computer (PCM) software needs to be updated – Low coolant level – Radiator fan does not turn off – Faulty intake air temperature (IAT) sensor

How the code P0128 is diagnosed:

The Torque app is being used to check the engine temperature. First and first, a simple check beneath the hood must be carried out: is the coolant level satisfactory? Check to see that the engine coolant temperature sensor connector is free of corrosion and that it is correctly connected. Do you know whether there are any service bulletins? The following step is to check to determine if the engine temperature sensor is functioning properly. You should check the engine temperature when the engine is cold if you have a scan tool that can record live data.

  1. Drive the car for 10 minutes and then check the engine temperature again; it should be between 180 and 220 degrees Fahrenheit (82 and 105 degrees Celsius).
  2. An ohmmeter can be used to test the engine temperature sensor if you don’t have access to a scan tool.
  3. More information may be found at: Symptoms, issues, and testing for the engine coolant temperature sensor are all covered in this article.
  4. It is common practice to replace a thermostat anytime the code P0128 is present.

In certain cases, reprogramming the engine computer (PCM) with current software may be sufficient to resolve the issue. This may be done at your local dealer and costs between $80 and $120 if the vehicle is not still under warranty. See the list of frequent issues below.

Common problems causing the code P0128 in different cars:

The code P0128 is frequently associated with a faulty thermostat in numerous General Motors vehicles (including the Chevrolet Trailblazer, Chevrolet Silverado, Chevrolet HHR, and other models). When the code P0128 is activated in some General Motors vehicles, the temperature gauge stops operating and the radiator fans operate continuously. Unless there are other concerns, replacing the thermostat is usually sufficient to resolve the problem. For the code P0128 in 2006 Chevrolet Malibu, Impala, Pontiac G6, and a few other vehicles, a 2007 General Motors Technical Service Bulletin proposes reprogramming the PCM as a remedy.

  • According to the Mazda service notice, the code P0128 may be caused by incorrect calibration of the PCM (engine computer) in some early Mazda 3 and Mazda 5 model year vehicles.
  • Subaru service bulletin 09-56-13 is available.
  • It was discovered that resetting the PCM (engine computer) is recommended when the code P0128 is detected in a handful of Chrysler technical service bulletins for many Jeep and Dodge models.
  • In this case, a shim should be used in conjunction with a thermostat housing that has been modified.

How the thermostat works

Thermostats are temperature-controlled valves that are fitted in the vehicle’s cooling system. It regulates the flow of coolant through the radiator and aids in and protects the radiator from overheating. Read more:Thermostat: how it works, what it looks like, what it causes, what it tests. Many contemporary automobiles have thermostats that are regulated electrically.

Jeep P0128

A Jeep P0128 –P0128-THERMOSTAT RATIONALITY fault code may be present on your Jeep, Dodge, or Chrysler product, among other things. A defective engine thermostat that opens too slowly or too rapidly is the most common cause of this problem. As part of determining whether to set the Jeep P0128 fault code, the PCM attempts to forecast what the engine coolant temperature should be during cold start-up based on the engine temperature and ambient temperature. When the anticipated temperature and the actual temperature from the engine coolant temperature sensor are compared, the PCM determines which value is correct (ECT).

For example, if the ambient temperature is between 17.6°F and 122°F and the engine coolant temperature is less than 122°F, the coolant temperature should reach 185°.

Read more: Jeep P0128? (TOP 5 Tips)

Diagnose Jeep P0128

To begin, check the level of coolant in the system. If it is too low, the P0128 error code will be generated. After that, use a noncontact infrared thermometer to check the thermostat’s functionality. Examine the difference between the ECT measurement and the real engine temperature. Alternatively, if the Jeep P0128 code persists after the thermostat has been replaced, check both the engine coolant temperature (ECT) and the ambient air temperature (AAT) sensor readings to ensure they are within an acceptable range based on the actual engine and ambient air temperatures.

The year is 2019.

Rick Muscoplat posted a blog entry on

P0128 Coolant Temperature Below Thermostat Regulating Temperature

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

What does that mean?

This diagnostic trouble code (DTC) is a general powertrain code, which means that it applies to any cars that are equipped with the OBD-II diagnostic system. Vehicles from a variety of manufacturers, including but not limited to Pontiac, Toyota, Mazda, Dodge, Chevrolet, Honda, Jeep, Ford, and Volkswagen, may display this code. Despite the fact that they are general, the particular repair processes may differ based on the make and model. If the engine’s powertrain control module (PCM) detects that the engine has not achieved the appropriate temperature level within a defined length of time after starting the engine, the engine will shut down.

Codes that are similar: P0125 When determining whether or not the engine has reached a ‘normal’ temperature, the length of time the vehicle has been running, the readings from the intake air temperature (IAT) sensor, the readings from the engine coolant temperature (ECT) sensor, and the speed of the vehicle are all taken into consideration.


With the exception of theMILillumination, you are unlikely to have any drivability issues. You may encounter symptoms such as the following in some cases:

  • When driving at faster speeds, the engine takes longer to warm up and the temperature of the engine lowers.

A typical thermostat is seen in the following photograph:


It is possible that a code P0128 indicates that one or more of the following has occurred:

  • Low engine coolant level
  • A leaking or jammed open thermostat (which is likely)
  • And a faulty alternator. A faulty cooling fan (which is operating excessively)
  • Sensor for coolant temperature (ECT) that is not working properly
  • The intake air temperature (IAT) sensor is faulty.

Possible Solutions

When working on the cooling system, make sure the engine is completely shut down and allowed to cool completely. When the engine is heated, the cooling system is pressured and extremely hot, and if you do not exercise caution, you might suffer burns or other bodily injuries. As a result of previous experience, it appears that the thermostat replacement is the most likely remedy for a P0128. However, the following are some recommendations for troubleshooting and correcting a P0128 OBD-II code:

  • Check the coolant’s strength level. Check to see that the cooling fan is operating properly (and that it is not running more than it should). If required, replace the item. Check that the engine coolant temperature (ECT) sensor is functioning properly, and replace it if required. Check the appropriate operation of the intake air temperature (IAT) sensor and replace it if necessary. If all of the foregoing parts are in excellent working order, the thermostat should be replaced. It is possible that the ECM may need to be reprogrammed to rectify the P0128 code in a Nissan car, therefore look for Technical Service Bulletins (TSBs) on the vehicle.

Any vehicle, regardless of its year, make, and model, should be inspected for Technical Service Bulletins (TSBs). If there is a recognized remedy established by the carmaker, it may save you both time and money throughout the diagnostic and repair process. If you own a Honda Accord, for example, you should search for TSBs, or technical service bulletins. Bulletin01-164, for example, may apply to you. Manufacturer communications are another term for technical service bulletins.

Other ECT engine coolant sensor and circuit related DTCs include, but are not limited to:P0115,P0116,P0117,P0118,P0119,P011A,P011B,P0125, andP0126. Other ECT engine coolant sensor and circuit related DTCs include, but are not limited to:P011A,P011B,P0125, andP0126.

p0128 DiagnosisRepair Video

We found this video to be really useful in diagnosing the p0128 error code. We have no affiliation with the creator of this video, and it is provided only for the convenience of our visitors:

Related DTC Discussions

  • P013A and P0128 codes on a 2007 Jeep Commander 5.7 Hemi are displayed. Those codes are being generated by my Jeep, but only in cold weather. When the Check Engine light illuminated for the first time, I had the coolant examined at the same time that my oil was changed. As a result of the somewhat low reading, the technician replaced the radiator cap and cleared the codes. It was until a few days later when the check engine light illuminated. 2006 Dodge Stratus is the vehicle in question. P0128 is the P0128 code for 2.4. As a result, I replaced the thermostat, housing, and coolant temperature sensor, which caused the code P0128 to appear. The identical error code appeared, and my car abruptly stopped while traveling. What else possibly be the source of my difficulties? P0128 and P0420 are trouble codes for the 2005 Chevrolet Optra. Good morning, gentlemen. My Optra has these issue codes, which I’m attempting to resolve on my own but need your assistance: I made the mistake of looking for explanations and possible remedies in DIY videos on YouTube. Initially, I attempted the Dura Lube extreme catalytic and exhaust treatment, as well as changing my gas from 87 to 91 octane and then driving the car
  • It was a 2002 Volvo V70 2.4 NonTurbo with 75k miles. P0128 temperature of the coolant The check engine light is illuminated. He comes up with the number PO128. The engine revs at stop lights and stalls on occasion when I’m traveling at a slow pace. When I came home one day, I saw that the fan was not turning on at all. I replaced the thermostat, and I’ll be changing the sensor later tonight as well. Is it possible that the revving has something to do with it? On a 2006 Chevrolet Trailblazer, the engine codes P0300, P0128, P0449, and P0601 are shown. After changing the alternator in my 2006 Chevrolet Trailblazer, I’ve been getting a lot of engine errors. P-0030, P-0128, P-0135, P-0449, and P-0601 are the codes to look for. It has a 4.3-liter V-6 engine. Could there be an issue that affects all of these codes in the same way? If it makes a difference, I also replaced the battery. Please accept my appreciation in advance. Doug
  • Codes P0410 and P0128 are related. Chevrolet Trailblazer (2005 model year) LSMy While cold and in drive, the 2005 Chevrolet Trailblazer makes a harsh noise as if it were burning rubber. It runs OK in Park, Reverse, and Neutral, but when in Drive, it makes a loud, vibrating sound and is difficult to drive. A code reader revealed that my check engine light was also illuminated, and that the two codes that were shown were P0128 and P04, with P0128 indicating that the cooling system temperature was below specifications and P04 indicating that the cooling system temperature was over specifications. Hello, The P0128 code on my 2001 Chevy Impala was pulled, and after following the required procedures, I discovered that I needed to repair the cooling fan and relay. Are there any tricks I should be aware of before I begin, and what is the complexity level of the game overall? Thank you for the helping hand. torc72
  • BMW problem codes from 2001 P0128 Hello, I have a 2001 BMW 325CI with a 2.5-liter engine. The P0128 light turned on a few days ago, and I reset the code, but the light came on again this morning. It is usual for the water temperature gauge to be half way up in the middle of the gauge while it is at this point. I look at the fan blade, and it appears to be turning. Can someone assist me with this situation, and if so, what can they do? The following are the duramax codes: 07 duramax codes p0087, p0128, p0672, p0673, 0676, p067107 duramax cabchassis and throwing codes: p087, p128, po087, p128, po672, po673, po676, p0671. The major source of worry is the po087 error code, which is having difficulty diagnosing the problem. Without the use of a scan tool, the problem was narrowed down to a bad cp3 fuel pump or a slight outside possibility that the rail pressure sensor was faulty. The vehicle has 135,200 miles on it. p0128 error code on a 2004 Chevrolet Silverado I have a 2004 Chevrolet Silverado with a problem code p0128. I’ve changed the thermostat and am still receiving the code, however it only occurs on frigid mornings when the temperature is low. It also appears to take a bit longer for the engine to warm up on mornings when the temperature is 40 degrees or below. There there a regulator or anything that could be cau
  • Is there something that could be cau
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Need more help with a p0128 code?

If you still need assistance with the P0128 error code, please ask your issue in one of our FREE vehicle repair discussion boards. Please keep in mind that this material is being provided solely for informational reasons. It is not meant to be used as repair advice, and we are not liable for any actions you take in relation to any vehicle. All of the information on this website is protected by intellectual property rights.

P0128 Fault Code Help please!

On August 24th, 2016, I made the decision to acquire a Solar Yellow 2016 Renegade Latitude. It had 32 miles on it at the time, perhaps 15 of which were logged by me when test driving it. I had no problems with my vehicle until January 9th of this year, when the check engine light on for the first time at about 4,000 miles. I took it to the dealer the next day, where I was told it was a thermostat/coolant issue, which was not a major concern, and they cleared the code. The light turned back on four days later, on January 13, when the power was restored.

  • I dropped in my Renegade for this problem as well as for my first oil change in a long time (mileage 4601).
  • The documentation claims that the malfunction indication bulb is illuminated, and that fault code P0128 has been registered, suggesting that a problem has been discovered in the thermostat system.
  • It took around 15 miles to get from the dealer to McDonald’s for breakfast and then back home.
  • I was driving my daughter to her father’s house, which is roughly one mile away from the dealership (I may add), when the light came on again at 5:10.
  • What are the remedies to this problem, because it is clear that a software upgrade was not the solution to this problem?
  • I bought a 2003 Jeep Liberty brand new with only 100 miles on the odometer.
  • I was not disappointed.
  • Any assistance, suggestions, or ideas would be much appreciated!

Need advice please – P0128 code and dealer explanation

I have a 2012 JKUR that was constructed in March 2012 and acquired it in April 2012. It has 25,000 kilometers on it. AEV 2.5-inch raise, 35-inch MTRs, exhaust spacers, axle gussets, trektop, and everything else is factory-installed. The significance for mentioning these minor modifications will become evident later on. This is a problem with the stock drivetrain, however it’s possible that I began the discussion in the incorrect place. After around 4 months, the P0128 error code began to occur.

  • As a result of my conversation with the dealer, they verified that the coolant temperature was too low for what they believed it should be, therefore the code.
  • Because this is my ‘work’ vehicle, it took several months before I was able to take it to the dealer.
  • The coolant temperature sensor and housing were changed by the dealer around two months ago.
  • The following day, the code failed again.
  • As a result of his consultation with Chrysler engineers, the service manager believes the pinion factor in the computer is incorrect for the size of tire on the jeep in question.
  • The factor in my case had never been accurate, and I pointed out that he should have tripped codes from the start because the factor was never proper in his thinking.
  • Maybe it didn’t run long enough to cause problems because I had the lift and tires installed inside the first 4000 kilometers.

I knew this was a load of baloney, but I decided to give it a go.

I had used the AEV plugin to modify the speedometer when I raised the car.

I reset it and adjusted my speedometer calibration, but the error message reappeared a week or so later.

Something is changing, and it’s becoming worse, in my opinion.

Despite the fact that I do not hear the famed 3.6 litre ticking sound, I have not conducted a leak down test.

I intend to meet with the dealer the following day to address the situation.

Based on my web research regarding the problem, I believe the thermostat is stuck or has failed completely.

My invoice from the last visit is missing, however I assume just the temperature sensor and housing were included in the total.

Is there anything else that anyone here can provide me in the way of guidance? Is it possible that I’ve missed anything, or are they speaking the truth? Thank you, and I apologize for the lengthy read.

P0128 Code

@Anybodyhome Have you had any further difficulties with this? The Sahara CEL on my wife’s car started working today. When she returned home, I checked the code, which was P0128. It is a 2015 model with 88K miles. Neither I nor anybody else knows if any of the coolant components (including the coolant) have been changed. We only purchased it a month ago and have already replaced all of the fluids, with the exception of the coolant. 1. On the trip home, the DIC registered as high as 226 points.

  1. 2.
  2. She stated that it appeared to be heating up normally.
  3. I’m not sure when the cooling fan kicks in, but I wouldn’t be able to tell because I wouldn’t be able to hear it.
  4. Even if the overflow is close at the maximum line, it is still heated.
  5. I’ll inspect the radiator when it has cooled down.
  6. 1.
  7. 2.
  8. Ours is hitting 226 before dropping down to a lower level.

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.

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).

  1. 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.
  2. Providing the thermostat is performing as it should, the next component to be checked is the engine coolant temperature sensing device.
  3. Check for reference voltage from the ECM to the ECT while the ignition is turned on and the engine is turned off.
  4. After that, start the engine and measure the voltage across the ECT signal line.
  5. However, because the values vary from car to vehicle, we will not go into specifics about what voltage you should be seeing here.
  6. 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.
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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.

P0128: Coolant Thermostat Temperature Below Regulating Temperature

When your check engine light illuminates as a result of trouble code P0128, it is possible that you have a problem with your engine thermostat. Here are some useful measures to follow in order to figure out how to solve error number P0128. Learn how to troubleshoot and repair your thermostat if you receive this error number. How to Interpret Your Check Engine Light

What Is Trouble CodeP0128?

This generic issue code can be triggered by a wide range of vehicle makes and models. When your powertrain control module detects an abnormal temperature measurement while your engine is running, a code is generated and displayed on your dashboard. When your thermostat does not read any temperature or reads the temperature incorrectly, you will often receive a notification from the system. In this case, the conclusion is formed based on the data from the intake air temperature sensor, the amount of time your vehicle has been operating, and the reading from the engine cooling temperature sensor.

According to your vehicle, the actual position of the thermostat, as well as the concerns associated with a malfunctioning thermostat, may be different.


When it comes to managing coolant temperature and monitoring for more significant engine concerns, your thermostat is a critical component. A faulty thermostat, on the other hand, is unlikely to result in any performance concerns. So, what exactly does the code P0128 represent? Most likely, you’ll see that your malfunction indication bulb has turned on. Additionally, while driving at high speeds, you may notice that it takes a longer period of time for your engine to warm up, or that the temperature drops.


One of the most common reasons for this is that the thermostat is leaking or has become stuck open. Due to the fact that related issues would normally result in another error code, this should be one of the first areas to look into. Your service handbook will have the exact position of your thermostat listed. Nonetheless, before you replace your complete thermostat, be sure to check the following locations as well: Low engine coolant might cause the engine operating temperature to rise to the point where the fault code P0128 is activated.

How toFix P0128

Begin with the most straightforward repair solution, which is low coolant levels. Before you begin, switch off your engine and allow it to cool completely before opening your coolant reservoir. A coolant system is pressurized when it is in use as it should be. If you don’t take caution when opening a hot coolant system, you might suffer burns and other damage. Once your engine and coolant system have cooled to a safe operating temperature, check the levels and strength of your coolant, and compare them to the levels indicated in the service manual for your vehicle.

It is possible that a defective thermostat will not show any apparent symptoms of harm. Check these other systems first before replacing your thermostat before replacing your thermostat:

  • The intake air temperature sensor, the engine coolant temperature sensor, and the cooling fan are all included.

P0128 Code Engine Thermostat Replacement 3.6L

The first time the weather got cold around here I had the check engine light show up on the dash. So I looked up the code using my Torque App on my tablet to see what it was. P0128 Coolant Temperature Below Thermostat Regulating Temperature. My first thought was ‘bad thermostat’, but with all the electronics involved now I needed to check things out a little bit more.What it means:In determining the engine did not reach a ‘normal’ temperature, it takes into account the length of time the vehicle has been running, the intake air temperature (IAT) sensor reading, the engine coolant temperature (ECT) sensor reading, and the speed of the vehicle.

Oh look the little red engine light on the dash.

P0128 code courtesy of the Torque App.

Kind of important to know when you are troubleshooting.On the 3.8L the thermostat is set for 176 degrees.

Though a couple references say 196, and a few replacements say 210 degrees.

Check the engine coolant level.

You might need to shine a flashlight into the bottle since they get a little discolored with age.

Nice thing about having the Torque App on my tablet, is I can see the sensor reading for Engine Coolant Temperature (ECT) and Intake Air Temperature (IAT).

Both of these tracked.

If you don’t have a reading on either of these then physically check the sensors.

The ECT is on the back left side of the engine (brake booster side).

I noted that my IAT sensor had a damaged shroud around the wiring that needed to be replaced.

It didn’t come on right away, and stayed off until the engine temperature got up to temperature.

Since pretty much everything else had been eliminated at this point, I started checking the upper radiator hose from the thermostat.

It wasn’t open yet.

Mine opened at 177 degrees.

I think I found my culprit.

Now shut the engine off and let everything cool down.

I just left mine till the morning.

It’s actually fairly easy to change out the thermostat.

Remove the engine cover.


Unscrew the two (2) bolts using a 10mm socket.

Pull the rubber radiator overflow hose out of the two (2) clips.

Use a flat tip screwdriver to loosen the two (2) hose clamps holding the intake tube to the throttle body and air box.

Carefully lift the hose until you can get to the IAT sensor on the bottom of the Intake Air Tube.

Mine was really stuck on their.

It kind of lays against the upper radiator hose so the shroud gets hot.


Not much to look at, but good time to make sure it’s not broken or caked full of stuff.

Mine had a little tab to lift and then it rotated out.


I wasn’t to surprised that I had a film of oil on the throttle body and butterfly valve.

You can wipe it off, just use a lint free rag and be careful.


Make certain the engine and coolant isn’t hot before you do this.

Place a bucket under the radiator drain and open the drain.

You can get to it from the front if you remove/already removed the plastic cover from between the bumper and grill.

guess I have another problem to look at.


Using a pair of channel lock pliers pinch the end of the hose clamps on both ends of the upper radiator hose.




Use a slight rocking motion to break the seal and pull it straight out.

Disassembly: Just a few pictures of the thermostat once it’s been removed.

Remove the rubber gasket from the houseing.


You will be able to see which way to turn it by looking at the tabs on the housing.

Remove the spring. 17.

Mine just fell out, but you may need to rock yours just a little to break the seal.

Inspect the houseing to make sure that there is no debris inside of it.

This is kind of optional.

Mine had some slime underneath it that I cleaned out with a cue-tip.

Just some comparison shots: The Mopar seemed to be of better quality than the aftermarket for the most part. Reassembly: 20.

Th point goes in first.

Install the new spring.


Line up the tabs with the slots on the housing and turn it clockwise until it seats in place.


Completed assembly.

Make sure the mating surface on the block is clean so that you can get a good seal.

Reinstall the two (2) bolts/screws holding the thermostat housing to the engine with a3 phillips screw driver, or a 10mm socket and extension.

Twist the upper radiator hose back down and slide the end over the thermostat housing.

Use a pair of channel lock pliers pinch the end of the hose clamps on both ends of the upper radiator hose.


Reconnect the harness to your IAT sensor.

Reconnect the intake tube to the throttle body and air box.

Reinstall the air inlet tube.



You will probably need to do this again when the engine warms up and the fluid starts flowing.


You are standing in front of it.


Close it when you get fluid.

Turn off the engine and refill your radiator as needed.

Reinstall your radiator cap.

Restart the Jeep and check for the thermostat to open at 210 degrees, or 204 degrees (This depends upon which thermostat you bought).


If you remove the radiator cap, make sure that it is not hot prior to removing.

Yep, made a little mess out of this one.

Contentand Design © 2002-present WanderingTrail,Ronald SeegertCommon Sense and Safety should always be observed when working on your vehicle or doing modifications.

You are responsible for your own installation, these write ups are a helpful guideline and should not be taken as an official installation instruction.

I try to keep the site up to date with changes that have occured as I discover them, but may not have the latest unless someone lets me know.

Only a few times have I needed to employe some actual help from a shop to get something done.

While I have spent many years working on mechanical systems, I am not an expert, nor do I pretend to be one.

I hope these write ups have been useful to you.All trademarked nameslogos are property of their respective ownersThis site is in no way associated with Stellantis. Jeep is a registered trademark of Stellantis.

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