Tool
Fuel trim reader
Fuel trims show how much the computer is adding or removing fuel to hold the right mixture. Add short and long term together to see the real correction.
Short answer
Add short term and long term fuel trim. A total within about plus or minus 10% is usually normal. Large positive numbers mean the computer is adding fuel because it sees a lean mixture; large negative numbers mean it is removing fuel because it sees a rich one.
Read your fuel trims
Take the numbers from your scan tool's live data. Bank 2 exists mainly on V and flat engines; leave it empty if your scan tool shows only bank 1.
Read the values with the engine fully warm. Use a minus sign for negative trims, for example -6.
The plus or minus 10% line is a rule of thumb. Limits vary by vehicle, and the computer usually sets a lean or rich code only well past it. Compare readings at idle and at about 2,500 rpm before you decide.
What fuel trim means
The engine computer aims for a set air-fuel mixture and checks the result with the upstream oxygen sensor. Fuel trim is the correction it applies to its base fuel calculation. Short term trim reacts within a fraction of a second. Long term trim is what the computer has learned over time and keeps after the engine is switched off.
Add both together to see the full correction. A positive total means fuel is being added because the mixture reads lean; a negative total means fuel is being taken away because it reads rich. When the computer runs out of room to correct, it sets a code such as P0171 or P0174 (system too lean) or P0172 or P0175 (system too rich).
Idle versus cruise
A trim that is high at idle and closer to normal at 2,500 rpm usually means unmetered air, such as a vacuum leak, because a small leak matters more when little air is flowing. A trim that stays high or gets worse at higher rpm and under load points more toward fuel delivery or the MAF sensor.