P0171 – System Too Lean (Bank 1)
Description
The ECM detects a lean air-fuel mixture on Bank 1 based on fuel trim and O2 sensor feedback.
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View Recommended ScannersSymptoms
- Rough idle
- Hesitation on acceleration
- Lack of power
- Poor fuel economy
- Possible misfires
Causes
- Vacuum leak at intake or hoses
- Dirty or faulty MAF sensor
- Weak fuel pump or clogged filter
- Leaking exhaust upstream of O2 sensor
Diagnosis
- Monitor short- and long-term fuel trims
- Smoke-test intake for vacuum leaks
- Check MAF readings vs RPM
- Measure fuel pressure under load
Repair
- Repair vacuum or intake leaks
- Clean or replace MAF sensor
- Repair fuel delivery issues
- Fix exhaust leaks before O2 sensor
