Long term vs short term fuel trim, explained
Short term fuel trim (STFT) is the engine computer's instant correction to the fuel it meters, moving many times a second in response to the upstream oxygen or air fuel sensor. Long term fuel trim (LTFT) is the learned correction: when short term trim keeps pulling the same way, the computer moves that amount into long term trim and stores it, so short term can return towards zero. For diagnosis, add the two per bank. The total is the whole correction, and a large total means the same thing whichever of the two is holding it.
How short term trim behaves
Short term trim follows the upstream sensor directly. With a narrowband oxygen sensor, which only says rich or lean, the computer steps and ramps the fuel back and forth across the switching point, so short term trim swings a few percent either side of its average all the time. With a wideband air fuel sensor, which reports how rich or lean, the correction is proportional and short term trim sits steadier. Either way, a single snapshot of short term trim tells you less than its average over a few seconds.
How long term trim learns
Long term trim changes slowly. When short term trim holds away from zero for a while, the computer shifts part of it into long term trim, a little at a time. Many strategies keep separate long term values for different ranges of rpm and load, which is why long term trim at idle and at cruise can differ. The learned values are kept in memory with the ignition off, and on many vehicles clearing codes or disconnecting the battery resets them, after which it takes a drive for them to relearn.
So the split between the two depends on timing. Just after a reset, the whole correction sits in short term trim. After a drive, most of it has moved into long term trim. The total barely changes, which is exactly why you add them.
Reading them together
- Short term near zero, long term at plus 14: a learned lean correction. The problem is real and has been there long enough to be learned.
- Short term at plus 14, long term near zero: the same correction, not learned yet. Either the memory was just reset or the condition just appeared.
- Short term at minus 10, long term at plus 10: a total near zero. Something changed in the last few seconds, such as a purge event or a change of load, and the trims are catching up.
- Totals that differ between banks on a V engine: something that belongs to one bank. See bank 1 vs bank 2 fuel trim.
Worked example: where a vacuum leak's correction sits
A vacuum leak on a 2.0 L inline four, from the model, after long term trim has learned most of the correction. The model assumes long term trim is learned per load cell, so each cell carries its own value.
| Cell | STFT | LTFT | Total |
|---|---|---|---|
| Idle | +1.5 | +13.4 | +14.9 |
| 2,500 rpm, no load | +0.5 | +4.9 | +5.4 |
| Cruise | +0.2 | +2.6 | +2.8 |
Short term trim alone reads +1.5 at idle and looks healthy. The total, +14.9, does not. Clear the codes on a vehicle that resets long term trim and, for the first minutes of driving, short term trim would carry the whole +14.9 instead. Same leak, same total, different split.
Common mistakes
- Reading short term trim alone and calling the engine healthy while long term trim holds the whole correction.
- Comparing long term trim at idle with long term trim at cruise as if they were one number. Many strategies learn them separately.
- Clearing codes before recording the trims, which on many vehicles throws away the learned values you needed.
- Judging a narrowband engine's short term trim from one frame while it swings. Average it over a few seconds.
- Mixing up the sign. Positive means the computer is adding fuel because the sensor says lean.
Practise it
- Every case in the daily drill shows short and long term trim per bank in each cell. Add them before you answer.
- Read how the model works for how the model splits the correction.
- Then try positive fuel trim at idle, the first shape most students meet.
Sources
Last reviewed . Standards and regulations are named so you can look them up; the service information for the vehicle always governs.
- SAE J1979, the OBD II diagnostic services standard, and its successor SAE J1979-2 for OBD on UDS. They define the fuel trim parameters (PIDs 06 to 09: short and long term trim for bank 1 and bank 2, scaled from minus 100 to plus 99.2 percent, where positive means fuel is being added) and the mass airflow parameter (PID 10, in grams per second).
- The US EPA onboard diagnostics requirement for light duty vehicles, 40 CFR 86.1806, which requires a fuel system monitor and largely adopts California's OBD II regulation, 13 CCR 1968.2.
- Manufacturer service information for the vehicle in front of you. It sets the real trim limits, when a lean or rich code is allowed to set, which side is bank 1, and the confirmation tests. These vary by make, engine and model year, so no number on this page replaces them.
- The TrimCase engine model, which computes every number in the worked example on this page.
TrimCase is a training aid built on a simulated engine model. It is not a diagnostic instruction for any real vehicle.
Frequently asked questions
Which matters more, short term or long term fuel trim?
Neither on its own. Add them per bank: the total is the whole correction. Long term shows what has been learned over time, short term shows what is happening right now.
What is a normal long term fuel trim?
Within a few percent of zero on a healthy engine. Many technicians treat a total of short plus long term trim within about plus or minus 10 percent as normal, but the real limits are the manufacturer's.
Does clearing codes reset long term fuel trim?
On many vehicles it does, and so does disconnecting the battery. Record the trims before you clear anything, and expect long term trim to need a drive to relearn afterwards.
Why does short term fuel trim jump around?
On an engine with a narrowband oxygen sensor the computer keeps stepping the mixture across the rich and lean switching point, so short term trim swings a few percent all the time. That is normal. Its average is what counts.