Guide

What is fuel trim?

Fuel trim is the percentage the engine computer adds to or removes from its basic fuel amount to keep the air fuel mixture correct. A positive value means it is adding fuel because the engine is running lean; a negative value means it is removing fuel because it is running rich.

Key facts

Key fact	Detail
What it is	The engine computer's fuel correction, shown in percent
Positive value	Adding fuel: the computer sees a lean mixture
Negative value	Removing fuel: the computer sees a rich mixture
Short term fuel trim (STFT)	Fast correction based on oxygen sensor feedback
Long term fuel trim (LTFT)	Slower, learned correction based on what short term trim has been doing
OBD2 PIDs	Service 01 PIDs 06 and 07 (bank 1), 08 and 09 (bank 2)
Related codes	Fuel trim limits exceeded can set P0170 to P0175, such as P0171 and P0172

How fuel trim works

In closed loop, the engine computer reads the upstream oxygen sensor. When the sensor shows a lot of oxygen in the exhaust (lean), the computer adds fuel; when it shows little oxygen (rich), it removes fuel. That fast correction is short term fuel trim.

If short term trim keeps correcting in the same direction, the computer moves the long term fuel trim, a learned value that shifts the basic fuel calculation so short term trim can return toward zero. Long term trim therefore shows what the computer has been doing over time.

How to read fuel trim values

What you see	What it means
Values near zero	Little correction needed
Positive (for example +12%)	The computer is adding fuel to correct a lean condition
Negative (for example -12%)	The computer is removing fuel to correct a rich condition
Long term trim large in one direction	A lasting lean or rich condition the computer has learned to compensate for

Look at short and long term trim together, since the total correction is the combination of both. Values and limits vary by vehicle, so compare with the service information for your car.

What pushes fuel trim lean or rich

Technical training material lists the same kinds of causes for each direction:

Lean (positive trim): air leaks such as vacuum leaks, clogged injectors or fuel filter, low fuel pressure, or air flow being measured too low.
Rich (negative trim): a leaking injector, a fuel pressure regulator problem, a slow oxygen sensor or a mass air flow sensor reading too high.

If the correction stays too large for too long, the computer sets a fuel trim code such as P0171 (system too lean, bank 1) or P0172 (system too rich, bank 1) and turns on the check engine light.

Watching fuel trim in OBD Master Pro

Fuel trims are among the live data sensors OBD Master Pro shows, along with oxygen sensor readings on the O2 sensor screen. With the engine fully warm, watch short and long term trim at idle and at a steady higher rpm, and note them in the freeze frame when a fuel trim code is stored.

Try it on your own car

OBD Master Pro turns your Android phone and a generic ELM327 adapter into a code reader: read and clear codes, view freeze frame and live data, and check emissions readiness.

Keep reading

Common codes: P0171 System Too Lean · P0420 Catalyst Efficiency · P0300 Random Misfire · P0455 EVAP Large Leak

All OBD2 guides · OBD2 by car brand · OBD2 trouble code guides

Sources

Published October 24, 2026. Updated October 24, 2026. General information only. Details generally vary by model, year and market, so always follow the owner's manual and service information for your vehicle.