Jacques Gordon has worked in the automotive industry for 40 years as a service technician, lab technician, trainer and technical writer. His began his writing career writing service manuals at Chilton Book Co. He currently holds ASE Master Technician and L1 certifications and has participated in ASE test writing workshops.
When diagnosing a check-engine light or drivability issue, you can learn a lot by looking at fuel trim data on a scan tool. Fuel trim can be affected by just about anything between the air filter and the muffler, including sensors, injectors, ignition, EGR, the engine’s mechanical condition and even the crankcase ventilation system. Fuel trim numbers alone won’t provide a complete diagnosis, but if you know what to look for, those numbers can lead you in the right direction.
Global OBD-II fuel trim reporting was standardized for all manufacturers in 2005 and later models equipped with a CAN bus control system. That’s what we’ll focus on here, but the same basic principles apply to all model years.
What is fuel trim?
The amount of fuel required to operate an engine correctly depends upon the amount of air flowing into the combustion chambers. Since the driver controls airflow (load) with the accelerator pedal, the powertrain control module (PCM) can only control fuel. It uses sensors to measure or calculate airflow, consults an air/fuel ratio map in its permanent memory, then chooses the correct injector pulse width to match that airflow. That pre-programmed injector pulse will provide exactly one gram of fuel for each 14.64 grams of air (a stoichiometric air/fuel ratio), but the PCM almost always adjusts injector pulse width to provide more or less fuel than specified in the map. That adjustment is “fuel trim.”
There are two types of fuel trim, long-term and short-term, and the numbers are displayed on the scan tool as a percentage of fuel added to or subtracted from the pre-programmed fuel quantity.
Volkswagen calls fuel trim “oxygen sensor adaptation,” and Ford classifies fuel trim as a “continuous monitor” that runs when the fuel control system is in closed loop operation. This shows us that fuel trim is an ongoing calculation based on data reported by the oxygen sensor. 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).
The formula for fuel trim calculation is:
Fuel mass = Air mass x (short-term fuel trim x long-term fuel trim) divided by (equivalence ratio x 14.64)
Understanding these terms and their relationship will help you understand how to use fuel trim data shown on a scan tool.
This is the desired air/fuel ratio, a command issued by the PCM. Greater than 1.0 is the command for a rich air/fuel ratio, and less than 1.0 is a command for a lean mixture.
Under steady-state conditions, this command constantly rises and falls to alternate between rich and lean mixture for proper catalyst operation. Many scan tools will display and graph this number in Global OBD-II.
Short-term fuel trim (STFT)
During normal closed-loop operation, the PCM uses short-term fuel trim (STFT) calculations to constantly command the air/fuel ratio slightly rich and then slightly lean. This is necessary for the catalytic converter to do its job correctly, and the average of the rich/lean swings will be in or near the middle of the oxygen sensor’s signal range. You can see this by displaying graphs of equivalence ratio, the oxygen sensor and STFT on a scan tool.
At idle the graphs will mirror or “chase” each other: they won’t have the same shape, but as one rises, the others will follow or fall.
Long-term fuel trim (LTFT)
As the engine ages and the cylinders don’t seal evenly anymore, the short-term fuel trim may trend high or low most of the time. The PCM has the ability to learn this trend and store it in memory, and it will use that number in the fuel trim calculation to compensate for the changes causing the trend. That’s long-term fuel trim (LTFT). Since fuel trim is an ongoing calculation, keeping one factor in the calculation constant makes it possible to return STFT to its normal range, enabling a faster and more accurate reaction to bigger changes in operating conditions, such as acceleration.
In general, it takes anywhere from five to 20 seconds for LTFT to change, and since it’s stored in memory when the engine is switched off, that same LTFT value is used next time the system goes into closed-loop operation.
These three variables in the fuel calculation — STFT, LTFT and equivalence ratio — are all generated by the PCM. The only other variable, air mass, is measured with a sensor.
To understand how the variables influence fuel trim, consider this: When the system is in open loop, there is no fuel trim because the PCM holds the three controlled variables at 1.0. The only variable that affects fuel requirement is airflow, and the calculation looks like this:
Fuel mass = Air mass x (1 x 1) divided by (1 x 14.64)
Some people think of LTFT as a coarse adjustment and STFT as fine adjustment. A more technical definition would be additive and multiplicative. Additive fuel trim calculations (STFT) won’t change much with engine speed or load. For example, a vacuum leak would produce an additive calculation because the effects of that leak increase very little with increasing engine speed. Multiplicative fuel trim calculations (LTFT) are greater as engine speed or load increases because, for example, the effect of a partially clogged injector increases as speed and load increase.