P0171
Generic (SAE)
System Too Lean (Bank 1)
Severity
Medium
Typical repair cost
30 – 600 EUR
System
Powertrain
What it means
The oxygen sensor reports more oxygen in the exhaust than expected, so the engine control module adds fuel. When the long-term fuel trim exceeds its limit (typically +25%), P0171 is stored.
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Symptoms
- Check engine light
- Rough idle, especially when cold
- Hesitation or stalling
- Poor fuel economy
Common causes
- Vacuum leak after the air flow sensor
- Dirty or failing mass air flow sensor
- Weak fuel pump or clogged filter
- Leaking PCV valve or hose
- Exhaust leak before the upstream O2 sensor
How to diagnose
- Watch long-term fuel trim at idle and at 2500 rpm: lean only at idle points to a vacuum leak
- Smoke test the intake
- Clean the mass air flow sensor with dedicated cleaner
- Check fuel pressure under load
- Inspect the exhaust manifold for cracks
Frequently asked questions
What is the difference between P0171 and P0174?
P0171 is bank 1, the side with cylinder 1. P0174 is bank 2 on V engines. Both together usually mean a shared cause like a vacuum leak or weak fuel pump.