P0171: System Too Lean (Bank 1) — Complete Guide
Key Takeaway
P0171 means too much air or not enough fuel. Here's every possible cause and how to find yours.
What "System Too Lean" Means
The air/fuel ratio should be ~14.7:1 (stoichiometric). P0171 means the ECM is adding more fuel than expected to compensate for excess air or insufficient fuel.
All Possible Causes
| Category | Specific Cause | Fix Cost |
|---|---|---|
| Air leaks | Vacuum hose cracked/disconnected | $0 – $200 |
| Air leaks | Intake manifold gasket leak | $200 – $500 |
| Air leaks | PCV valve stuck open | $20 – $50 |
| Air metering | MAF sensor dirty or failed | $0 – $400 |
| Air metering | Air filter not seated (unmetered air) | $0 |
| Fuel delivery | Weak fuel pump | $400 – $800 |
| Fuel delivery | Clogged fuel filter | $50 – $150 |
| Fuel delivery | Leaking fuel injector | $150 – $300 |
| Exhaust | Exhaust leak before O2 sensor | $100 – $400 |
| Sensors | O2 sensor failing | $150 – $300 |
How to Diagnose P0171
1. Check for vacuum leaks — listen for hissing, spray carb cleaner around intake (RPM changes = leak).
2. Clean the MAF sensor — $8 MAF cleaner spray from auto parts store.
3. Check fuel pressure — should match factory spec (usually 35-65 PSI).
4. Inspect the PCV valve — should rattle when shaken.
ProHow Steer Helps
Steer reads fuel trim data and identifies whether P0171 is an air leak issue (high LTFT) or a fuel delivery issue (dropping fuel pressure).
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