OBD-II · Engine and powertrain (P)
P0156
Oxygen sensor circuit bank 2 sensor 2
What it means
The downstream (after the catalytic converter) lambda sensor signal on bank 2 is invalid. Check whether it is an electrical fault or a real mixture problem before replacing parts. Confirm bank 2's location in the engine's service information.
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 downstream sensor |
| 02 | exhaust leak |
| 03 | wiring or connector damage |
| 04 | open signal or sensor ground circuit |
| 05 | water in connector |
What to check, step by step
- 01Check the connector and wiring along the exhaust for melting, chafing or water in the plug
- 02Check the warm downstream sensor against OEM live-data specifications; a working catalyst normally suppresses rapid switching, so a steady rear signal alone is not a sensor fault.
- 03Look for exhaust leaks before the sensor (manifold gasket, cracked flex pipe)
- 04Check the signal and ground wires for continuity and shorts back to the ECU
- 05Replace the sensor with the correct OE-type part if the wiring is good