Applicable Systems
OBD-II
Symptoms
Fuel trim for bank 1 exceeds +25% (lean); ECM adding maximum fuel; possible vacuum leak or low fuel pressure.
Possible Causes
- Vacuum leak at intake manifold gasket
- PCV valve stuck open
- Low fuel pressure (pump/filter/regulator)
- MAF sensor under-reading airflow
Troubleshooting Steps
- Step 1: Check STFT/LTFT (should be < +/- 15%)
Trims within range after repair - Step 2: Smoke test for vacuum leaks
No leaks found - Step 3: Check fuel pressure and volume
Pressure and volume within spec - Step 4: Clean/replace MAF sensor if under-reading
MAF reading accurate - Step 5: Replace PCV valve if stuck open
PCV valve operates correctly
How to Reset
Clear with OBD-II scanner; reset trims and perform drive cycle.
Common Replacement Parts
- PCV Valve (OEM spec)
- Fuel Filter (if equipped)
Frequently Asked Questions
- Q: Common causes of P0171?
Top 3: vacuum leak, dirty MAF sensor, low fuel pressure. Start with smoke test, then MAF cleaning, then fuel pressure check.
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Source: SAE J2012 standard DTC list + OBD-II portal
Category: OBD-II
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