Applicable Systems
OBD-II
Symptoms
Fuel trim for bank 1 exceeds +/-25% adaptation limit; ECM cannot maintain stoichiometric mixture.
Possible Causes
- Vacuum leak causing lean condition
- Fuel pressure low causing lean
- MAF sensor inaccurate causing wrong mixture
Troubleshooting Steps
- Step 1: Check STFT and LTFT for bank 1
Trims within +/- 10% - Step 2: Perform smoke test for vacuum leaks
No leaks found - Step 3: Check fuel pressure at rail
Pressure within spec (35-55 psi typical) - Step 4: Verify MAF sensor reading accuracy
MAF reading matches expected - Step 5: Repair root cause and clear fuel trims
Trims return to +/- 5%
How to Reset
Clear with OBD-II scanner after repair; reset fuel trims and drive cycle.
Frequently Asked Questions
- Q: What is fuel trim?
STFT (Short Term) = real-time adjustment. LTFT (Long Term) = learned adaptation. Positive trim = adding fuel (running lean). Negative = removing fuel (running rich).
Source: SAE J2012 standard DTC list + OBD-II portal
Category: OBD-II