What is fuel trim? (Solution)

What is Fuel Trim? Fuel trims are what engine control system use to compensate for all problems relating to air-fuel ratios (known as λ or lambda,) and combustion. The Electronic Control Module (ECM) or Powertrain Control Module (PCM) uses a few sensors to determine how much air is flowing into the engine.

  • Basically, fuel trims are the percentage of change in fuel over time. For the engine to operate properly, the air:fuel ratio needs to stay within a small window of 14.7:1. It has to remain in this zone under all the various conditions an engine encounters every day: cold start-up, idling in heavy traffic, cruising down the highway, etc.

What is normal fuel trim?

Assuming that the engine is in excellent mechanical condition, and that all sensors implicated in metering both intake air and fuel, short-term fuel trim values should generally be between positive 10%, and negative 10% when the engine is running at a steady speed.

What is a fuel trim issue?

Diagnostic Trouble Code (DTC) P0170 stands for “Fuel Trim Malfunction (Bank 1).” Fuel trim is the adjustment made by the PCM to the engine’s air-fuel mixture. The P0170 code indicates that the fuel trims have reached an abnormally low or abnormally high value for a specified amount of time.

What causes high fuel trim?

What is fuel trim? Extra oxygen in the exhaust indicates a lean air/fuel mixture, so the PCM increases injector pulse width to add more fuel (positive fuel trim). Too little oxygen in the exhaust indicates a rich mixture, causing the PCM to reduce injector pulse width (negative fuel trim).

How do you reset fuel trims?

This is done by disconnecting the battery, waiting for 10 minutes or so, and/or turning the ignition key to the starter position to discharge the system.

Can fuel trim cause misfire?

Yes, the -35 percent fuel trim could be required to pull the signal into range, causing the lean misfire.

How do you diagnose fuel trim?

To test a rich response, you can feed some propane vapor from a small propane tank into the throttle body or a vacuum hose connection on the intake manifold. This time, you should see a drop in fuel trim readings, with STFT going NEGATIVE, and LTFT creeping downward in response to the rich fuel mixture.

What causes a negative fuel trim?

Negative fuel trim means the fuel mixture is too rich and the car’s computer is cutting back on the amount of fuel it’s delivering to the engine to compensate. This can be caused by any of the items in a long list of causes for rich mixture.

What does fuel Trim system Lean Mean?

This code could either mean that the oxygen sensor is detecting too much oxygen or too little fuel in the mixture. This lack of fuel is known as a lean mixture. If you wish to learn more about rich and lean OBD codes, you can read our discussion about air-fuel mixture.

What causes lean fuel trim?

The lean condition can be caused by a vacuum leak, which introduces more air into the air-fuel mixture, or by a weak fuel system, which does not input enough fuel into the air-fuel mixture.

Is fuel trim bad?

Fuel trims (LTFT, STFT, or LTFT+STFT) exceeding ±10% may indicate a problem in the engine or the sensors.

What affects long term fuel trim?

Long-term fuel trim relies on sensors further downstream in the exhaust system, past the catalytic converter. While the sensors’ signal will fluctuate frequently upstream, where short-term fuel trim is measured, the signal at the sensors further down should be fairly constant.

How much is too much fuel trim long term?

How much LTFT is too much? Each car maker is different, but the general rule is that a LTFT of +25% or -25% indicates a serious problem.

What does SHrtFT1 mean?

SHrtFT1 2.6 – means the short-term fuel trim is at 2.6%. ( read more about short and long term fuel trim and live OBD2 data) Long FT1 -3.0 – the oxygen sensor or O2 sensor has not switched yet to long-term fuel trim for the fuel injectors.

How do I reset my long term fuel trim HP tuner?

while you’re logging, open the vcm controls, hit reset fuel trims. your LTFT’s will go to zero as soon as you do it.

What Is Fuel Trim?

Automobile Repair Library, Auto Parts, Accessories, Tools, Manuals and Books, Car BLOG, Links and Index are some of the resources available on this website. byLarry Carley (c)2019 AA1Car.com All rights reserved. Fuel Trim refers to the adjustments made to the fuel mixture by the engine computer (PCM) in order to maintain a balanced air/fuel ratio. When using a scan tool, the fuel trim data is often shown as a PERCENTAGE reading. The engine computer strives to maintain the fuel mixture balanced between 14.7 and 1 in order to produce the least amount of pollutants (14.7 parts of air to one part fuel).

A rich fuel combination can create greater power (up to a point), but it also increases fuel consumption and pollutants, which are both undesirable.

In some cases, a low fuel mixture can reduce fuel consumption while increasing emissions, particularly if the air/fuel combination is so lean that it fails to ignite and produces lean misfire.

Oxygen sensors are fundamentally two types of indicators: RICH and LEAN.

  1. During rich operation (too much fuel and not enough air), the oxygen sensor provides a higher voltage signal that informs the engine computer that the engine is receiving too much fuel and that it should reduce the amount of fuel delivered to the engine.
  2. It is necessary to have an accurate feedback signal from
    the oxygen sensor in order for the fuel trim values to be accurate; otherwise, the engine computer has no means of knowing whether the fuel mixture is running rich or lean.
  3. While the fuel feedback control system is in ‘closed loop’ until that point has been reached and reached, the fuel mixture is locked at a predefined value, and no fuel trim changes are made.
  4. Whenever the engine is turned off, the fuel trim settings are saved in the computer’s memory so that the next time the car is driven, it may continue from where it left off.

Erase the computer’s memory using a scan tool or unplug the battery or PCM power source to remove codes. This will also delete the fuel trim values, which means the computer will have to learn the fuel adjustments all over again the next time it is started up again.

How to Read Fuel Trim

When you insert a scan tool into the OBD II diagnostic port, which is situated under the instrument panel, you may view the fuel trim value (on the drivers side near the steering column). In only a few seconds after turning on the ignition, the scan tool will be configured and ready to connect with the vehicle’s onboard computer. The scan tool may require the car’s year, make, model, and engine VIN code to be entered before it can read the data. This will vary depending on the scan tool and the vehicle in question.

  • Depending on the scan tool and how its menu choices are configured, you can select the option that allows you to read live data from the computer’s operating system.
  • There will be two fuel trim values for inline four and six cylinder engines on this list, and four fuel trim values for V6 and V8 engines on this list as well (one pair for each cylinder bank).
  • According on engine load, speed, temperature, and other operating circumstances, this number fluctuates fast and can bounce around quite a bit.
  • In the case of long-term fuel trim (LTFT), it is a longer-term average of what the engine computer has been doing to balance the fuel mixture over a period of time that is defined.

STFT B1is Short Term Fuel Trim engine cylinder Bank 1STFT B2is Short Term Fuel Trim engine cylinder Bank 2LTFT B1is Long Term Fuel Trim engine cylinder Bank 1LTFT B2is Long Term Fuel Trim engine cylinder Bank 2LTFT B1is Short Term Fuel Trim engine cylinder Bank 1LTFT B2is Long Term Fuel Trim engine cylinder Bank 2LTFT B1is Short Term Fuel Trim engine cylinder Bank 1LTFT B2is Long Term

What Fuel Trim Values Mean

Positive fuel trim readings indicate that the engine computer is adding fuel to the engine (by raising the pulse width or on-time of the fuel injectors) in order to provide more fuel to it. For want of a better term, it is making an attempt to RICHEN the fuel mixture because it believes the engine’s air/fuel ratio is running too lean. It is possible to have a negative (-) fuel trim value if the engine computer is removing fuel from the engine by lowering the pulse width or on-time of the fuel injectors.

This is done in order to LEAN out the fuel mixture in order to compensate for what the engine perceives to be a rich operating state during operation.

If the O2 sensors indicate that the car is running low on fuel, the computer adds fuel and provides a POSITIVE fuel trim value for the driver.

While your engine is running, you may use a scan tool to examine the STFT and LTFT fuel trim readings to determine if the air/fuel combination is running rich (negative fuel trim percentages) or lean (positive fuel trim percentages) (positive fuel trim percentages).

What Fuel Trim Values Should Be

When the engine is idle or being kept at a constant RPM, the STFT and LTFT should be within a few percentage points of zero, in the ideal case, but not always. Keep in mind that STFT can bounce about quite a bit, especially when you rapidly open the throttle or slow the vehicle. LTFT, on the other hand, can tell you if the average fuel/mixture is running rich or lean on average. As near to zero as feasible is ideal, yet they can range between 5 and 8 percent depending on the state of the engine.

LTFT levels that rise to about 20 to 25 percent are typically associated with the lean codes P0171 or P0174.

The STFT value for this scantool is 25 percent, as indicated on the display.

As soon as the engine starts and enters closed loop operation, the fuel trim values will begin to fluctuate and become inaccurate.

How Fuel, Ignition and Engine Problems Affect Fuel Trim

Generally speaking, lean fuel mixes are more prevalent than rich fuel mixtures, however either can occur depending on the underlying cause. With LEAN fuel mixes, your scan tool will produce POSITIVE fuel trim values that are higher than usual compared to normal. Fuel combinations with a high concentration of ethanol will provide NEGATIVE fuel trim readings.

Possible causes of LEAN fuel mixtures include:

Lumps in the intake manifold, particularly close to the throttle body and at vacuum hose connections are signs of air or vacuum leakage. Fuel pump that is not capable of providing sufficient pressure or volume Restriction on the flow of fuel (like a pinches hose or plugged filter) A faulty fuel pressure regulator that is unable to maintain enough fuel pressure is to blame. There are air leaks in the PCV plumbing system. The MAF (Mass Airflow) sensor is dirty and is under-reading the amount of airflow entering the engine.

Possible causes of RICH fuel mixtures include:

Fuel injector with a leak High fuel pressure caused by a malfunctioning fuel pressure regulator or a clogged fuel return line Air filter that is extremely unclean or air intake system that has restrictionsExhaust system that has restrictions (clogged converter, crushed exhaust pipe or plugged muffler) The oxygen sensor is not working properly (output shorted to voltage so it reads RICH all the time)

Using Fuel Trim to Diagnose Problems

Diagnose vacuum and fuel delivery leaks with the help of Fuel Trim. Take a look at the Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) readings when the engine is running at idle. The normal range can be as large as plus or minus 8, although it is better if it is closer to zero. If the values for STFT and LTFT are greater than 10, this indicates that your engine is operating LEAN. Increase the engine’s speed to 1500 to 2000 RPM and maintain it for about a half-minute or more. The presence of a vacuum leak at idle is confirmed if the fuel trim figures return to a more typical value.

  • The presence of a lean fuel situation is more likely to be caused by a fuel delivery problem (poor fuel pump, clogged fuel filter, filthy fuel injectors, or a leaky fuel pressure regulator) than than by a vacuum leak, especially if the fuel trim readings do not move considerably.
  • One or more fouled spark plugs that are misfiring on and off might be the source of the problem, as could a weak ignition coil or a faulty plug wire that is causing some intermittent misfires.
  • Fuel trim can be used to determine whether or not your fuel injectors are unclean.
  • The most likely reason for this would be clogged fuel injectors.
  • The solution in this case is to clean the injectors.
  • It is possible that the lean fuel condition is caused by low fuel pressure or air/vacuum leaks if the fuel trim values do not change after the injectors have been cleaned.
  • During the idling period of the engine, momentarily detach a vacuum hose.
  • Feeding propane vapor into the throttle body or a vacuum hose connection on the intake manifold can be used to test for rich response.
  • In reaction to the rich fuel mixture, you should notice a decrease in fuel trim readings, with STFT becoming NEGATIVE and LTFT becoming increasingly negative.

When you produce an artificially low or rich fuel mixture, and there is no change in the fuel trim readings, this indicates that the engine computer is not running in a closed loop, or that the oxygen sensor(s) are not responding to changes in the fuel mixture.

Click Here to Download or Print This Article.

Ratios of air to fuel Wells Manufacturing has developed a fuel trim (PDF file). P0171P0174 Ford P0171P0174 Codes of Conduct for Lean The Effects of Fuel Injection on Emissions The Most Frequently Encountered Error Codes (and what causes them) Sensors for measuring mass airflow Oxygen Sensors are a type of sensor that detects the presence of oxygen. Sensors with a wide ratio of air to fuel (WRAF) More information about Check Engine Lights Code of Fault Diagnostics Diagnostics of the Scan Tool Onboard decoding is available.

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Make sure to check out our other websites as well: You Can Do Your Own Auto Repair CarleySoftware OBD2HELP.com Random-Misfire.com Help using the Scan Tool TROUBLE-CODES.com

Understanding Short Term and Long Term Fuel Trims

today’s world, the notion of fuel trims, as well as the way an engine control unit (ECU) employs fuel trims to maintain a stoichiometric balance between fuel and air in petrol engines, is one of the least understood components of modern engines and fuel management systems. In contrast, the process of cutting the air/fuel mixture in order to maximize power while simultaneously saving fuel and reducing emissions is very uncomplicated, given that the issue is handled logically. Using the above topic as a jumping off point, we will quickly examine what fuel trims are, why they are necessary, and how to use fuel trims as diagnostic assistance in this post.

What are fuel trims?

It takes all of the available air to burn all of the fuel in a petrol engine that is running on a stoichiometric air/fuel combination (14.7 parts air to 1 part fuel). However, because all petrol engines require more (or less) fuel as the load placed on them varies, the balance between air and fuel must inevitably alter in order to accommodate variations in the demand for fuel as the load is changed. To put it
another way, fuel trims are the continuous adjustments an ECU makes to fuel delivery methods in order to keep the air/fuel mixture as close as possible to the stoichiometric point (also known as Lambda = 1) throughout the engine’s operational range.

The oxygen content of the exhaust stream is monitored by an oxygen sensor located upstream of the catalytic converter, and the voltage generated by the sensor is directly proportional to the amount of oxygen present in the exhaust flow.

Notably, the signal voltage fluctuates from around 0.22 to approximately 0.09 volts in most applications, which the ECU interprets as fluctuations in the amount of oxygen present in the exhaust stream.

In practice, a signal voltage of 0.45 indicates that the air/fuel combination is at or near the stoichiometric value, while signal voltages above or below this value indicate that the mixture is either rich or lean.

Please keep in mind that on some applications, most notably on some General Motors products, the oxygen sensors are electrically opposite, which means that a low signal voltage indicates a rich mixture, as opposed to the lean mixture that a low signal voltage would indicate on most other applications.

In most cases, the signal voltages generated by these sensors are not directly related to fuel delivery strategies, although there are some exceptions to this rule.

More to the point, the pattern of changes in the signal voltage of the downstream oxygen sensor is compared to the pattern of changes in the signal voltage of the upstream oxygen sensor, and the ECU calculates an efficiency value for the catalytic converter based on the differences or similarities between the signal voltage patterns of the two oxygen sensors.

The signal voltage from the downstream sensor, on the other hand, should remain relatively constant around the mid-point of the voltage range that applies to that sensor, assuming that the catalytic converters’ efficiency is around 75% or greater.

Consequently, in the absence of any defects, errors, or malfunctions that may effect fuel trims, long-term fuel trim values indicate an average of the trims/adaptations that the ECU has made to adjust the air/fuel mixture over a predefined period of time.

How to interpret fuel trim data

Throughout this article, we will assume that all oxygen-, or air/fuel ratio sensors function in the same way, in the sense that a low signal voltage indicates a lean mixture, and that a high signal voltage indicates a rich mixture, and that this is correct. We’ll also assume that the engine is in good mechanical condition, that there are no engine vacuum leaks, that there are no misfires, and that there are no exhaust leaks upstream of the oxygen sensors, among other assumptions. The fuel trim data for both the short- and long-term will be provided as percentages if you attach a scan tool to such a vehicle.

While the indicated amount for long-term fuel cuts can be as high as 6 to 8 percent (depending on the application) in some situations, it can be either a negative or a positive number in others.

First, let us consider the positive numbers: Fuel trim values that are positive As a result, if the displayed fuel trim value is a positive figure, it indicates that the ECU is increasing the injector pulse width in order to add more fuel to an already lean air/fuel mixture in order to enrich the mixture, as indicated by the data it is getting from the input sensors.

Please keep in mind that some scan tools may report fuel trims as Lambda = X, where ‘X’ may be a value bigger or lower than ‘1,’ with ‘1’ denoting a stoichiometric air/fuel combination.

It’s important to remember that fuel trim values are only trustworthy if it’s known that the oxygen -, or air/fuel ratio sensors are completely functioning and operating in closed loop mode, and that there are no air/fuel metering or ignition related codes saved in the computer.

This can be the result of a malfunction, or it may occur as a result of someone having recently deleted all fault codes from the vehicle’s computer.

So what should fuel trim values be?

However, while zero percent fuel trim values are ideal, there is no such thing as a flawless engine, which means that in reality, maintaining zero percent fuel trim values on a constant basis becomes increasingly difficult as an engine ages. In spite of the fact that fair wear and tear on engines is unavoidable and modern ECUs are programmed to compensate for increased oil consumption rates, sensors losing sensitivity, and other factors that affect fuel trims, modern ECUs have a limited capacity to compensate for some factors, so keep the following in mind when interpreting fuel trim values: – Fuel trim settings for the short term Under normal operating conditions, assuming that the engine is in excellent mechanical condition and that all sensors involved in metering both intake air and fuel are functioning properly, short-term fuel trim values should generally range between positive 10% and negative 10% when the engine is running at a constant speed.

It should be noted, however, that because abrupt fluctuations in engine speed can cause short-term fuel trim values to vary dramatically, all fuel trim values should be obtained at a minimum of three consistent engine speeds, which should be at idle, at around 2500 RPM, and at approximately 3500 RPM.

Fuel trim values throughout the long run When the engine is running at a constant speed, long-term fuel trims should be at, or near to, 0 percent under ideal circumstances.

If the long-term fuel trim values fluctuate in a pattern that is similar to the short-term fuel trim values, consider a faulty catalytic converter (see below for further information).

While there may be a problem when long-term fuel trims are more than 10 percent or so to either side of zero percent, it is important to note that codes indicating rich or lean running conditions will generally only be set when the deviation reaches about 25 percent or more to either side of the zero percent mark.

However, once the variation exceeds around 25% when the engine is operating at a constant speed, there is an issue that will nearly always be signaled by a rich or lean running trouble code in the instrument cluster.

How to use fuel trim values as diagnostic aids

As previously indicated, fuel trim numbers that differ by a few percentage points from the optimum 0 percent are not always symptomatic of major difficulties with the vehicle. Although no codes have been set as a result of the deviation, fuel trim values (both long and short) that vary from modest deviations to around 25 percent on each side of 0 percent are obviously worthy of consideration. Listed below are some ideas and tactics to help you detect the most frequent faults/defects/failures/malfunctions that result in lean air/fuel mixes and hence high positive fuel trim values:

  • Small vacuum leaks
  • If both the short and long term fuel trim readings are greater than about 10% with the engine at idle, raise the engine speed to around 2000 RPM for approximately 30 seconds. The presence of a tiny vacuum leak whose effects are lessened or removed at high engine speeds can be determined if the fuel trim values recover to a more normal level after this period. The volume of air traveling across defective MAF sensors might be overstated, resulting in lean operating conditions. A fuel pressure that is insufficient
  • Check fuel pressure using a specific fuel pressure gauge. If your fuel injectors are clogged or filthy, do a spray pattern and volume test to see how well they are working. Sensor(s) for oxygen, air/fuel ratio, or other parameters that are marginally faulty
  • Depending on how much time has passed since the sensor was last used, the sensor may become stuck at reading lean circumstances as a consequence of the sensor not responding to switching signals from the ECU, or the sensor may be slow to respond to switching signals.
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The following are some pointers and strategies for diagnosing the most frequent faults/defects/failures/malfunctions that result in rich air/fuel ratios and high negative fuel trim values. –

  • Excessive fuel pressure
  • Check fuel pressure using a specialized fuel pressure gauge
  • Excessive fuel pressure
  • Fuel injectors that are leaking
  • Run a spray pattern and volume test on the injectors to see how well they are working. The volume of air traveling across faulty MAF sensors might be underestimated, resulting in rich running conditions. Exhaust leaks should be repaired as soon as they are discovered because exhaust gas contaminates the ambient air that oxygen sensors use as a reference. One or more cylinders have experienced a loss of compression. Because of the lack of compression, combustion is weak or partial, resulting in unburned hydrocarbons deceiving oxygen sensors into indicating a rich mixture. Marginal misfire
  • Not all misfires are severe enough to result in the setting of a misfire code, therefore inspect or replace suspicious spark plugs, and use an oscilloscope to examine the performance of all ignition coils. Sensor(s) for oxygen or air/fuel ratio that are marginally faulty
  • Due to a failure to respond to switching signals from the ECU, sensors may become stuck at reading rich circumstances
  • Alternatively, sensors may be slow to respond to switching signals.


The accurate interpretation of fuel trim values, as well as an understanding of their ramifications, may provide a technician with an almost surefire method of diagnosing faults and difficulties that would otherwise take hours to identify and resolve. The truth is that fuel trim values give you valuable information about the general health of your engine and fuel system in a way that few other diagnostic procedures can equal, so take use of this information.

Fuel Trim: How It Works and How to Make It Work For You

Jacques Gordon has almost 40 years of experience in the automotive sector, having worked as a service technician, lab technician, trainer, and technical writer among other positions. Chilton Book Company was where he began his writing career, creating service manuals. The ASE Master Technician and L1 certificates are now in his possession, and he has participated in ASE test writing seminars. When diagnosing a check-engine light or drivability issue, looking at fuel trim data on a scan tool can provide a great deal of useful information.

  • Even though fuel trim figures alone will not give a thorough diagnosis, they can point you in the correct way if you know what to look for.
  • That’s what we’ll be focusing on here, although the same fundamental concepts apply to all model years as they do to the previous ones.
  • The amount of fuel required to properly operate an engine is determined by the amount of air that enters the combustion chambers during the combustion process.
  • It does this by using sensors to detect or compute airflow, consulting an air/fuel ratio map stored in its permanent memory, and then selecting the appropriate injector pulse width to match the measured or calculated airflow.
  • The term for this change is ‘fuel trim.’ In general, there are two types of fuel trim: long-term and short-term.
  • Fuel trim is referred to as ‘oxygen sensor adaptation’ by Volkswagen, while fuel trim is classified as a ‘continuous monitor’ by Ford, both of which operate when the fuel control system is in closed loop operation.
  • Extra oxygen in the exhaust signals a lean air/fuel combination, prompting the PCM to increase injector pulse width in order to add more fuel to the mixture (positive fuel trim).

In order to calculate fuel trim, the following formula is used: Fuel mass = Air mass x (short-term fuel trim x long-term fuel trim) divided by 100.

Ratio of equivalence This is the required air/fuel ratio, which is a directive given by the power conversion module (PCM).

If the system is in steady-state, this command rises and falls continuously to alternate between a rich and a lean mixture in order to ensure appropriate catalyst activity.

Short-term reduction in fuel consumption (STFT) As part of regular closed-loop operation, the PCM makes use of short-term fuel trim (STFT) computations to maintain a consistent air/fuel ratio that alternates between being slightly rich and slightly lean.

Using a scan tool, you can observe graphs of the equivalency ratio, the oxygen sensor, and the STFT, which are all related.

Long-term reduction in fuel use (LTFT) As the engine ages and the cylinders no longer seal equally, the short-term fuel trim may tend to be high or low for the majority of the time, depending on the circumstances.

That is a long-term reduction in fuel use (LTFT).

This allows for a faster and more accurate response to larger changes in operating circumstances, such as acceleration.

The PCM is responsible for generating the three variables used in the fuel calculation: the STFT, the LTFT, and the equivalency ratio.

Consider the following examples to better understand how factors affect fuel trim: During open loop operation, there is no fuel trim since the PCM maintains the three regulated variables at their default values of 1.0.

The terms additive and multiplicative would be used in a more technical sense.

In the case of a vacuum leak, an additive calculation would be produced because the consequences of the leak grow very little with rising engine speed.

This is because, for example, the effect of a partly blocked injector becomes more apparent as engine speed and load increase.

The fuel trim should be checked for at least 30 seconds at three distinct engine speeds: idle, 1,500 rpm, and 2,500 rpm, according to the manufacturer’s recommendations.

In the event that everything is stable and functioning properly, the fuel trim numbers should be no larger than 10%, and the overall fuel trim numbers should be no greater than 10% when the figures are combined together.

In this case, LTFT is 12 percent and STFT is negative 6 percent, resulting in total fuel trim of 6 percent.

Because the PCM adjusts for normal wear on an older engine, the LTFT is normally a little higher on an older engine.

Whatever the engine, whether it’s an older model with a basic oxygen sensor or a modern one with a wide-band air/fuel ratio sensor, LTFT will continue to shift as needed in order to keep the STFT swings within acceptable ranges.

The LTFT can shift a surprising amount, but when it reaches a value of plus or minus 25 percent, the MIL will light and a code will be set. The following are the fault codes associated with fuel trim:

  • P0170: fuel trim bank 1
  • P0171: system too lean (bank 1)
  • P0172: system too rich (bank 1)
  • P0173: fuel trim bank 2
  • P0174: system too lean (bank 2)
  • P0175: system too rich (bank 2)
  • P0176: fuel trim bank 3
  • P0177: fuel trim bank 4
  • P0178: fuel trim bank 5
  • P0179: fuel trim bank 6
  • P0180: fuel trim bank 7
  • P0179:

By the time LTFT reaches 25 percent of the market, there will be additional codes as well. While it is possible to acquire valuable information from the scan tool even if the LTFT is below that limit, whether or not other codes are present, it is preferable to attach additional test equipment to validate your diagnosis. What is causing the high fuel trim numbers? In this case, the PCM believes that the air/fuel ratio is too lean and it adds fuel to bring the STFT control range back to the proper range.

This opens the door to three possibilities:

  • It is possible that unmeasured air is entering the combustion chambers
  • The amount of fuel reaching the combustion chambers is less than the amount specified. One or more sensors is reporting improperly
  • One or more sensors is reporting wrongly

When pondering what may be causing each of these situations, the first thing to explore is how the PCM decides how much air is being circulated. If the engine is equipped with a mass airflow sensor (MAF), high fuel trims at idle are a typical indicator of a vacuum leak, especially if the engine’s long term fuel consumption (LTFT) drops as engine speed increases. Because the volume of air passing through the vacuum leak does not increase with engine speed, the leak has less of an impact on the air/fuel ratio at higher speeds and loads.

  1. Keep in mind the many ‘calibrated vacuum leaks’ such as crankcase ventilation, the evaporative emissions (EVAP) purge valve, and, if installed, air-shrouded injectors while looking for vacuum leaks.
  2. Because the additional air (pressure) in the manifold is still recorded by the MAP sensor on engines that employ a manifold absolute pressure (MAP) sensor to estimate airflow, a vacuum leak has no effect on fuel trim on these engines.
  3. It’s important to remember that the PCM does not detect fuel flow; it just knows the injector pulse width and thinks that the fuel is being delivered as instructed.
  4. If you add propane and see that the LTFT and equivalency ratio readings drop, there is most likely an issue with the fuel delivery system.
  5. Don’t forget to check this at a variety of speeds and loads, since fuel flow issues are frequently not visible at idle.
  6. When determining fuel trim, the PCM interacts with all of the oxygen sensors in the system.
  7. In both the short and long term, a catalyst code will have an impact on fuel trims.
  8. What is causing the low numbers?

If the LTFT, also known as total fuel trim, is more than 10 percent negative, the PCM believes the air/fuel ratio is too rich and leans out the mixture calculation in order to bring the STFT control to the proper range of operation. This opens the door to three possibilities:

  • There is insufficient air reaching the combustion chambers. The amount of fuel reaching the combustion chambers is greater than the amount specified. One or more sensors is reporting improperly
  • One or more sensors is reporting wrongly

An example of anything that inhibits airflow into the cylinders is a broken catalytic converter, which results in excessive exhaust back pressure. At idle, this can cause the fuel trims to move in opposing directions, resulting in a positive STFT and a negative LTFT, respectively. On a scan tool, you can observe indications of excessive exhaust back pressure; predicted load will be low at wide-open throttle (WOT), and fuel trims will trend negative as engine speed increases. The most visible source of extra fuel is a leaky injector, which is most noticeable while the engine is running at idle.

  • Excessive crankcase vapors or a flooded EVAP canister can also give the appearance of excessive fuel, especially when the engine is running at idle.
  • Changing the oil may frequently indicate this problem by restoring the fuel trim numbers to their original values.
  • Negative LTFT values will appear if the vehicle is located at an elevation greater than a few hundred feet above sea level.
  • One method is to check that the total fuel trim is within 10% of its maximum and then go for a test drive.
  • The PCM may need to be taught the ‘new normal’ over a few miles and/or cold starts, but it’s a fascinating process to see.
  • When you start the engine with all of the fuel trims set to zero, pay attention to the short-term fuel trim setting.
  • If STFT begins to rapidly rise into the double digits, there is still a problem with the system.
  • Take note of the differences in reactivity between MAF systems and MAP systems.
  • There’s nothing like first-hand experience with known-bad and known-good automobiles when it comes to diagnostics, and this is true for everything.

Note from the author: We would like to thank Snap-on Diagnostics for their cooperation in the preparation of this article.

What are fuel trims all about?

Dale Toalston is an ASE Certified Technician who wrote this article. On the forums, there are frequently issues about fuel trims, and I’ve had a few letters from people asking for explanation on the subject. There are those who may have seen the fuel trim PIDS (parameter ID) on their scan tool datastream and wondered what they were for. This article will explain. So, what exactly are fuel trims, and what are they used for? Hopefully, we have been able to clear up any misunderstandings. A thorough grasp of gasoline trims can result in a more rapid diagnosis and the identification of potential problems with your car in the future.

  1. The air-to-fuel ratio must be maintained within a narrow window of 14.7:1 in order for the engine to run effectively.
  2. The engine computer attempts to maintain the optimum air-to-fuel ratio by adjusting the amount of fuel that is introduced into the combustion chamber.
  3. The oxygen sensors may be compared to the ‘eyes’ of the computer, which are constantly monitoring the mixture of oxygen in the exhaust.
  4. If the oxygen sensors notify the engine computer that the exhaust mixture is too lean, the computer compensates by increasing the length of the injector pulse, or ‘on-time,’ in order to add fuel.
  5. Fuel Trim is the term used to describe the addition or removal of fuel from a tank.
  6. Changes in the voltages of the oxygen sensors result in a direct change in the amount of fuel.
  7. In order for the long term fuel trims (LTFT) to be effective, the short term fuel trims must be effective.
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A negative percentage for fuel trim implies that fuel is being taken away, whereas a positive percentage shows that fuel is being added.

As you travel the steep terrain, you may find yourself ascending and descending several hundred feet at a time on a temporary basis.

It’s the same with short- and long-term fuel-saving measures.

Normally, a normalSTFT value will range between negative and positive single digits in most cases.

Although they usually hover in the positive or negative 5 percent range, they can occasionally rise to 8 or 9 percent, depending on the efficiency of the engine, the age of the components, and other considerations.

Under typical conditions, it should be near to zero, positive or negative single digits, or even negative double digits.

STFT Readings in the Normal Range If you have ST or LT fuel trims that are in the double digits, either positive or negative, you should consult your mechanic.

If you have a fuel injector leak, an unmetered air leak, or something similar, you should investigate it.

STFT Reading with a Lean Effort It will begin to ascend instantly, indicating that the computer has started adding gasoline.

The computer will keep increasing the amount of gasoline added until the leak has been repaired completely.

After a period of time, the LTFT will begin to represent this relative increase in fuel.

It will continue to add fuel until the STFT reaches its maximum calibration, which is generally 25%.

If an engine is running rich as a result of a fuel leak, the result is the inverse of this (P0172,P0175).

For example, if an oxygen sensor was stuck in the rich position, the computer would think it was reading properly and begin removing gasoline to make up for the difference.

It appears like the engine is running rich based on the codes, but in reality, it is running lean.

In addition, each bank has its own readout for the fuel trim.

When operating a V-style engine, you can tell which bank is running rich or lean by looking at the fuel trims for that bank.

With your newfound understanding of gasoline trims, you can eliminate the guesswork involved in determining the state of your fuel system, and you may even be able to save some of your hard-earned money.

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.

What Should Long Term And Short Term Fuel Trim Be?

If you purchase a product after clicking on one of our affiliate links, The Drive and its partners may get a commission. More information may be found here. Your car’s engine is a remarkable piece of engineering, but it only takes a small amount of damage to cause problems with the way it operates. Things might get hazardous if there is too much air in the engine room, not enough fuel in the tank, or a field mouse making its way into the engine area. Fortunately, there are various sensors, as well as your car’s electronic control unit (ECU), that are in charge of monitoring the entire process, and a failure in any element of the system might result in problems for you.

Gasoline trim is a procedure that occurs both short- and long-term that allows the ECU to keep track of how much fuel is being pumped into the engine.

However, it is not too late to become familiar with the ins and outs of fuel trim!

The Drive’s fluid dynamicists have done the mathematics to offer you the fundamentals of fuel trim, including why and how it occurs, as well as what you can do to correct the situation if there is one.

What Is Fuel Trim?

To put it another way, fuel trim refers to the changes that a vehicle’s engine control unit (ECU) makes to the fuel delivery system in order to keep the air-fuel combination as close to its optimal ratios as feasible while driving.

Short Term Fuel Trim

It is possible to experience short-term fuel trim as a result of fluctuations in the oxygen content of exhaust gases. An oxygen sensor continuously analyzes the flow and provides a signal that is proportional to the amount of oxygen present in the gases being monitored. In a fully operating system, the ECU will react to changes in the exhaust gases and make modifications as soon as they are detected. Because of this fast response, it is referred to be short-term.

Long Term Fuel Trim

In order to provide long-term fuel trim, sensors that are further downstream in the exhaust system, past the catalytic converter, are used. While the signal from the sensors upstream, where short-term fuel trim is recorded, will change regularly, the signal from the sensors further downstream should remain rather consistent. Long-term fuel trim refers to the measurements taken by these sensors, which are essentially an average of the alterations performed by the ECU as a result of short-term fuel trim in the first place.

That Sounds Like A Ton Of Data. What Does It Mean?

Your car is always checking its air-fuel mixture and making modifications to maintain it as near to the optimal range as possible, so you really need all of the information you can get your hands on about it.

A scanning tool may display the short- and long-term fuel trim values as a % on both the short- and long-term. This can also be a negative number in some cases. Here’s what it implies in practical terms.


As a result, when the value is a positive number, the ECU is adding more fuel to the mixture in order to enrich it since the information it is getting suggests that the air-fuel combination is too lean.


Alternatively, if the number is negative, it indicates that the ECU is lowering the flow of fuel into the injectors as a result of data it has received indicating that the air-fuel combination is excessively rich.

What Causes Lean Or Rich Mixtures?

It might be annoying to try to figure out what is causing your low or rich fuel blends, but it is not impossible. In general, if you’re experiencing problems with the air-fuel combination, it’s likely that one of these factors is to blame.

Fuel System Problems

Because to a problem in the fuel system, the levels of gasoline entering the engine might be too high or too low, resulting in a rich or lean condition, depending on the situation.

Oxygen Sensor Failure

The failure of an oxygen sensor may result in the ECU being unable to make the proper judgments regarding the air-fuel mixture.

Mass Air Flow Sensor Failure

The mass airflow sensor measures and transmits the amount of oxygen entering the engine at any given point in time. The failure of a sensor might result in the ECU delivering incorrect fuel mixes.

Leaks and Damage

A disruption in the air-fuel mixture may occur if the system is anticipating a specific quantity of fuel or air and does not receive it when expected.

Fuel Trim Terms You Need to Know

Learn something new!

Air-Fuel Mixture

In order for your car’s engine to function correctly, it requires a certain mixture of air and fuel. The air-fuel mixture is referred to as the air-fuel mixture ratio, and it is monitored by a number of sensors. The electronic control unit (ECU) in the automobile is the brain, and it is responsible for controlling the mixture and making adjustments to the delivery of either component depending on its evaluations.


Running rich indicates that there is an excessive amount of gas present in the air-fuel combination. This can occur for a variety of reasons, but in a typically operating system, the ECU will instruct the engine to reduce the amount of gasoline being delivered to compensate.


Running lean indicates that the engine doesn’t have enough fuel to keep up with the amount of oxygen that it’s receiving from the environment. Higher fuel levels in the mixture should be made possible by allowing the ECU to open the fuel injectors more widely.


The term ‘running lean’ refers to an engine that does not have enough fuel to match the amount of oxygen it is getting. Higher fuel levels in the mixture should be made possible by the ECU being able to open the fuel injectors more.

FAQs About Fuel Trim

If you have questions, The Drive has the answers!

Q:Ok, Then Can I Make Changes To The Fuel Trim?

A:Flashing (wiping and reprogramming) the ECU may be one method of accomplishing this, but there is no practical value to tinkering with the fuel trim or the air-fuel mixture unless you’re trying to boost performance in a competition situation. It’s always preferable to have a professional diagnose and correct any issues with your fuel trim operations or the air-fuel mixture.

Q:So Will I See A Check Engine Light?

A:likely It’s that you will.

A malfunctioning sensor or a change in the air-fuel ratio detected by the ECU will most likely result in the check engine light turning on in your vehicle.

Q:Alright, Can A Fuel Trim Issue Cause Other Problems?

As a result, you may encounter backfires, stalling, and other problems while driving if your engine isn’t getting the fuel it needs to work efficiently. This is problematic for a variety of reasons, but it’s especially difficult to ignore the possibility that it may cause you to crash your automobile.

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