OBD-II · Engine and powertrain (P)
P0138
Oxygen sensor high voltage bank 1 sensor 2
What it means
The downstream lambda sensor signal is higher than expected. 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
- usually no change in how the car drives
- may fail the emissions part of the roadworthiness test
Likely causes, most likely first
| No. | Cause |
|---|---|
| 01 | rich running |
| 02 | faulty downstream sensor |
| 03 | signal short to voltage |
| 04 | oil or coolant contamination |
| 05 | open sensor ground circuit |
What to check, step by step
- 01Read fuel trims: if the ECU is removing fuel, find the rich cause first (purge valve, injectors, fuel pressure)
- 02Check for fuel-diluted oil and for unexplained coolant loss that could contaminate the sensor
- 03Check the signal wire for a short to voltage, including a heater-to-signal short inside the sensor
- 04Replace the sensor if the mixture and wiring are fine