OBD-II DTC · POWERTRAIN · Severity: MEDIUM
P0171: System Too Lean (Bank 1)
ECM has added more than +20% to +25% combined Short-Term and Long-Term Fuel Trim on Bank 1 to maintain stoichiometric λ = 1.0.
| DTC Code | P0171 |
|---|---|
| Standard Definition | System Too Lean (Bank 1) |
| Subsystem | POWERTRAIN |
| Urgency / Severity | MEDIUM |
| Standard Reference | SAE J2012 / ISO 15031-6 |
1. Symptoms
- Rough idle or hesitation on acceleration
- Elevated Long-Term Fuel Trim (LTFT > +15%)
- Hard cold starts
2. Common Root Causes
- Unmetered air leak (vacuum hose, intake manifold gasket, PCV valve)
- Dirty or contaminated Mass Air Flow (MAF) hot-wire sensor
- Low fuel rail pressure or restricted fuel filter/injector
3. Diagnostic Procedure
- Check STFT and LTFT at idle vs 2,500 RPM (if fuel trim improves at RPM, suspect vacuum leak)
- Perform smoke test on intake tract and PCV hoses
- Clean MAF sensor element with dedicated MAF cleaner and check fuel rail pressure