OBD CodesWhat it means
"Lean" means too much air for the fuel being burnt. The computer has added as much extra fuel as it is allowed to and the mixture is still lean. Either unmeasured air is getting in, or not enough fuel is arriving.
Codes in this group
| Code | Meaning |
|---|---|
| P0171 | Mixture too lean, bank 1 |
| P0174 | Mixture too lean, bank 2 |
| P2187 | Mixture too lean at idle, bank 1 |
| P2189 | Mixture too lean at idle, bank 2 |
Bank 1 is the side of the engine that has cylinder 1. Sensor 1 is before the catalytic converter and sensor 2 is after it. More terms explained.
What you may notice
- Rough idle
- Hesitation, most of all when cold
- Lack of power
- Occasional misfire
Common causes
Most likely first:
- A vacuum leak: a split hose, intake gasket or loose oil filler cap
- A dirty mass air flow sensor reading low
- Weak fuel pump or blocked fuel filter
- Dirty or partly blocked injectors
- An exhaust leak ahead of the first oxygen sensor
What to check first
- Listen for hissing around the intake with the engine idling
- Check all small rubber hoses and the oil filler cap and dipstick are seated
- Replace an overdue fuel filter
- Both banks lean (P0171 with P0174) points to something shared, like the air flow sensor or fuel supply
Have a mechanic confirm. A fault code tells you which system reported a problem, not which part has failed. The same code can have several causes, and they differ between car models. Use this page to understand the code, then have a qualified mechanic test the car before any part is replaced.
After the repair
Once the cause is fixed, the code can be cleared with a scan tool, or the warning light goes out by itself after several trips without the fault. Clearing a code without fixing the cause only hides it for a while. More about clearing codes.
Related codes
Mass air flow (MAF) sensor (P0100), Manifold pressure (MAP) and barometric sensor (P0105), Intake air temperature sensor (P0110), Engine coolant temperature sensor (P0115), Thermostat and engine warm-up (P0125), Throttle position sensor (P0120), Accelerator pedal sensor and throttle agreement (P2120), Electronic throttle actuator (P2100). All codes.
Written in November 2026. These are generic OBD-II codes, the same on all makes. Causes and repair steps differ between models.