OBD-II · Engine and powertrain (P)
P0139
Oxygen sensor slow response bank 1 sensor 2
What it means
The downstream lambda sensor changes too slowly.
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 | aged sensor |
| 02 | exhaust leak |
| 03 | sensor contamination |
| 04 | wiring resistance |
| 05 | catalyst storage issue |
What to check, step by step
- 01Fix any misfire or fuel-trim codes first
- 02Check for exhaust leaks near the sensor
- 03Use the OEM rear-sensor response test and specified conditions; account for the catalyst's oxygen-storage delay rather than judging response to a brief throttle blip.
- 04Look for contamination sources: oil burning, coolant loss, silicone sealant, fuel additives
- 05Replace the sensor only if response remains outside specification after heater, wiring, exhaust leaks and mixture faults are ruled out.