P0170: Fuel Trim Malfunction — Bank 1
What to do
Expect worse economy and possibly hesitation. Ask for the fuel trim values to be read at idle and at higher load — that comparison is what points at the cause.
What this code means
The module's fuel corrections on bank 1 have gone outside the range it can work within, without settling clearly into lean or rich. Fuel trim is the running record of how far the module has had to deviate from its base fuelling map to keep the mixture correct — so a trim fault is a statement that something upstream is wrong, not that the fuelling system itself has failed. Read the actual short- and long-term trim values before anything else; their sign and how they change with engine load narrow the cause faster than any part swap.
Likely causes
Start here
- Vacuum leak in intake manifold or hoses · moderate
- Contaminated or failing mass air flow sensor · easy
- Faulty oxygen sensor · easy
- Low fuel pressure · moderate
Less common
- Clogged fuel injectors · moderate
- Exhaust leak before the sensor · moderate
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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