P0171
PowertrainFuel Systemhigh severitySystem Too Lean Bank 1
What P0171 Means
The long-term fuel trim for Bank 1 has exceeded the positive limit, indicating the engine is running lean. The PCM is adding more fuel than normal to compensate.
Symptoms
- Check Engine Light
- Rough idle
- Hesitation on acceleration
- Poor fuel economy
- Possible misfires
- Hissing sound from vacuum leak
Common Causes
- Vacuum leak (intake manifold gasket, brake booster hose, PCV valve)
- Low fuel pressure
- Dirty or faulty MAF sensor
- Exhaust leak before O2 sensor
- Faulty O2 sensor
- Clogged fuel injector
Diagnostic Steps
- Check fuel trim values at idle and 2500 RPM
- Perform smoke test for vacuum leaks
- Test fuel pressure at idle and under load
- Clean or test MAF sensor
- Check for exhaust leaks near manifold
- Check PCV system
Where to start: Most common: vacuum leak. Perform smoke test first. Check MAF sensor. Test fuel pressure.
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