Engine
What are fuel trims and how do you read them?
Fuel trims are the percentage the control unit adds to or removes from injection to keep the mixture right according to the oxygen sensor. A positive value means the engine gets too little fuel for the air coming in (lean) and a negative one that it gets too much (rich). When the trim reaches its limit, codes such as P0171 or P0172 are stored.
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Short-term or long-term?
Short-term trim changes constantly with the oxygen sensor signal. Long-term trim learns the deviation that keeps repeating and applies it all the time, so short-term trim returns close to zero.
What matters is the sum of the two: it says how much the control unit is compensating.
How are they used to diagnose?
Compare at idle and at about 2,500 rpm with a hot engine. If the mixture is lean at idle and improves with rpm, it is usually a vacuum leak. If it is lean across the range, it points to a lack of fuel or an airflow sensor that reads low.
If one bank corrects a lot and the other does not, the problem is on that bank: an injector, a leak in its manifold or its sensor.
Which values are normal?
Small values, close to zero, indicate an engine in good shape. The limit at which a code is stored is set by each manufacturer; the service information gives the reference for each engine.
General guidance only: every manufacturer and model has its own details. Always follow your vehicle's manual and, if in doubt, ask a workshop.
Related
Related trouble codes
- P0171Mixture too lean (bank 1)
- P0172Mixture too rich (bank 1)
- P0174Mixture too lean (bank 2)
- P0175Mixture too rich (bank 2)
- P2187System (bank 1): too lean at idle
- P2188System (bank 1): too rich at idle
Related symptoms
- Car uses more fuel than normal: causes and what to check
- Rough idle: why the engine shakes or hunts when stationary
- Black smoke from the exhaust: causes and what to do
Learn more
See what your car is saying
With the ECO-LITE, your workshop sees the fuel trims live, remotely, and can record them during a road test.
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Frequently asked questions
What is a lean mixture?
More air than there should be for the fuel injected. The control unit makes up for it by adding fuel.
Do diesels have fuel trims?
Not in the same way: diesels run with excess air and regulate the injected quantity differently.
Does clearing codes erase the learned trims?
On many cars yes, and the control unit learns them again while driving.