Fuel trim is a window that allows you to see what the computer is doing to control fuel delivery and determine how the PCM's adaptive strategy is operating.

Why was fuel trim created?

In order for vehicle manufacturers to comply with EPA emissions regulations, catalytic converters were added to reduce tailpipe emissions. Catalytic converters need a stoichiometric air/fuel ratio of approximately 14.7:1 to obtain the greatest emissions reductions.

Vehicle engineers designed closed-loop engine control systems to maintain that ratio, adjusting injector pulse width based on information from die oxygen sensor and other inputs.

Short-term fuel trim (STFT) and long-term fuel trim (LTFT) are expressed as a percentage, and the ideal range should be within ±5%.

Positive fuel trim percentages indicate that the powertrain control module (PCM) is attempting to richen the fuel mixture, to compensate for a perceived lean condition. Negative fuel trim percentages indicate the PCM is attempting to lean out the fuel mixture, to compensate for a perceived rich condition. STFT and LTFT percentages are the adjustments made by the PCM to maintain the 14.7:1 ratio.

No matter what the driveability issue happens to be, the fuel trim window should be used first to check the STFT and LTFT parameters.

There are two basic fuel control systems used on most vehicles: speed density systems, which use rpm, manifold absolute pressure (MAP) and barometric pressure (BARO) to calculate engine load, and
mass airflow systems, which use the mass airflow sensor (MAF) and rpm. to calculate engine load.
In both cases, the PCM begins with a standard injector pulse width calculation, based on various inputs and internal fuel cell tables.

The equation used by early speed density OBD II vehicles to establish initial pulse width is: Injector Pulse Width = (RPM × MAP/BARO) × TPS × ECT × IAT × Battery Volts × O2 (Short Term x Long Term).
Once the vehicle is running and the engine control system enters closed-loop, the PCM relies primarily on feedback from, the oxygen sensor to determine if the stoichiometric air/fuel ratio is being maintained.

Think of closed-loop operation as a Sense-Decide-React sequence.
Once the system enters closed-loop, the Sense phase begins, and is handled by the oxygen sensor. In the Decide phase, the PCM uses the oxygen sensor data to determine if the proper 14.7:1 air/fuel ratio is being maintained.

If the ratio is correct, the PCM decides that no change should be made to the injector pulse width. In this scenario, the React phase maintains the same injector pulse width. However, if the air/fuel ratio is 16.1:1 (lean) during the Sense phase, the PCM makes the decision to increase the injector pulse width to correct the lean air/fuel ratio condition.

In the React phase, the PCM commands the fuel injector to stay open longer. The Sense-Decide-React sequence continues throughout closed-loop operation, maintaining the proper air/fuel ratio.

During closed-loop operation, the PCM reports changes in fuel trim calculations via the OBD II generic data parameters short-term and long-term fuel trim. STFT for most vehicles will normally sweep rapidly in response to the oxygen sensor.

In many cases, if you graph Bank 1 STFT and BlSl O2 sensor, you'll see the oxygen sensor go rich and STFT go lean to adjust the air/fuel ratio. The oxygen sensor will then go lean and STFT will go rich.

LTFT for most vehicles will remain more stable, adjusting over a longer period of time. On some vehicles, if STFT has reached the specified limit, LTFT will change in a few seconds. On other vehicles it may take 15 to 20 seconds before a change occurs.

The LTFT calculation is normally kept in memory, so the PCM is ready to use the last known injector pulse width following a restart. STFT will normally begin at 0% and adjust to the current conditions. Both STFT and LTFT will normally reset when all trouble codes are cleared.

Performing an injector balance test is simple, as long as you can gain access to the fuel injector connectors. Unplug one injector at a time and wait until the maximum LTFT change is reached.
On some vehicles you'll use STFT for this test, or a combination of both STFT and LTFT. With one fuel injector unplugged, the oxygen sensor will see a lean condition and the PCM will compensate by,increasing the pulse width of the functioning injectors. to reach stoichiometry.

The results of this particular test, with injector No. 1 unplugged the LTFT change is approximately + 14%, injector 2 +10%, injector 3 +17% and injector 4 +16%. Injectors 3 and 4 contribute a greater volume of fuel than injectors 1 and 2. We know this because the amount of fuel trim increase is greater with these injectors unplugged.

Injector 2 is the cause for concern; with injector 2 unplugged, the remaining injectors need to supply only +10% total.
This injector may have a slight leak that's causing the negative fuel trims. A new set of injectors fixed this vehicle.

This scanner screen capture is using the meters mode of our scanner

Notice the averages look perfect so you assume all is OK but look further and see what the maximum values were for long and short term fuel trims

Notice how lean the LTFT is and fuel trim learn is enabled and that the minimum for that drive cycle for both long and short terms at time are super rich

Here using 8 meters we see when engine is in decel and fuel learn is disabled as throttle is closed as lifting the gas pedal. The negative torque value assures engine was commanded for decel

Long term is reporting rich yet short term shows otherwise

If you look at the scanner data in charts mode you realize that perfect fuel trims average overall did not tell the whole story as you see here any time engine is at idle the LTFT only report way too lean and fuel learn is enabled

This then clearly shows there is some type of unmetered air flow problem such as a leak between the aircleaner and throttlebody.
This shows you must spend the effort to analyze the scanner data you bothered to do a recorded scanner testrun

Notice how LTFTs show super lean yet STFTs report normal ?

This shows a different problem.
Notice the LTFTs are mostly way too lean in almost all engine conditions.

This would be like bad E10 gas, exhaust leak or bad O2 sensors