OBD-II diagnostic trouble code
P0171Caution
System Too Lean (Bank 1)
The engine is getting too much air relative to fuel on bank 1 — the computer is already adding extra fuel to compensate (positive fuel trim) but still can't reach the correct mixture. Left alone, a lean mixture runs hotter and can damage the catalytic converter or even the engine over time.
Symptoms
- Rough idle or a slight surge/hunting at idle
- Hesitation or lack of power during acceleration
- Occasional misfire, especially under load
- Slightly higher fuel consumption despite the lean condition (the ECU is over-fueling to compensate)
Likely causes
- Vacuum leak — cracked hose, worn intake gasket, or a loose connection letting in unmetered air
- A dirty or failing mass airflow (MAF) sensor under-reporting incoming air
- Weak fuel pump or clogged fuel filter not delivering enough fuel pressure
- A clogged or partially blocked fuel injector on bank 1
How to diagnose it
- Check short and long-term fuel trim values with a scan tool — trims consistently above +10% confirm a lean condition
- Spray a small amount of carb cleaner around intake hoses/gaskets while idling and watch for an RPM change, which reveals a vacuum leak
- Check fuel pressure at the rail against spec, and inspect the MAF sensor for dirt or damage
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Code names are compiled from open/standardized SAE and ISO references. Explanations, symptoms, causes and fixes are original. Covers generic (P0/C0/B0/U0) codes only — manufacturer-specific codes are planned for a future update.
Frequently asked questions
Can I drive with P0171?
You can usually keep driving with P0171, but don't put it off — the underlying cause can get worse or lead to further damage the longer it goes unaddressed. Get it diagnosed as soon as you reasonably can.
How much does it cost to fix P0171?
Typical repair costs for {code} range from roughly 50 to 600 EUR, depending on the specific cause and your car — see the fixes listed above for a breakdown.
What causes P0171?
Vacuum leak — cracked hose, worn intake gasket, or a loose connection letting in unmetered air A dirty or failing mass airflow (MAF) sensor under-reporting incoming air Weak fuel pump or clogged fuel filter not delivering enough fuel pressure
Reviewed by Artyom SemenovAutomotive Editor · Fact-checked by Yauheni Kapliarchuk, Editor-in-Chief