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
Typical fixes & cost
- Repair or replace a leaking vacuum hose or intake gasket50–250 EUR
- Clean or replace the MAF sensor60–220 EUR
- Replace fuel filter, or fuel pump if pressure is confirmed low150–600 EUR
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.
AS
Reviewed by Artyom SemenovAutomotive Editor · Fact-checked by Yauheni Kapliarchuk, Editor-in-Chief