OBD-II · Engine and powertrain (P)
P0131
Oxygen sensor low voltage bank 1 sensor 1
What it means
The upstream lambda sensor signal is stuck lean or too low. Monitor criteria depend on sensor type and OEM: distinguish an electrical voltage fault from an in-range lean/rich signal before diagnosing mixture causes.
Symptoms
- engine warning light
- higher fuel consumption
- possible hesitation or uneven running
- may fail the emissions part of the roadworthiness test
Likely causes, most likely first
| No. | Cause |
|---|---|
| 01 | vacuum leak |
| 02 | low fuel pressure |
| 03 | exhaust leak |
| 04 | faulty lambda sensor |
| 05 | signal wire short to ground |
What to check, step by step
- 01Read fuel trims: if the ECU is adding a lot of fuel, find the real lean cause first
- 02Smoke-test the intake for vacuum leaks, including the crankcase breather/PCV valve
- 03Check for exhaust leaks before the sensor that draw in outside air
- 04Check the signal wire for a short to ground
- 05Substitute a known-good sensor if the mixture and wiring are fine