P0158
PowertrainO2 Sensor Circuit High Voltage (Bank 2 Sensor 2)
The post-cat oxygen sensor on bank 2 sits in the exhaust after the catalytic converter, and its job is to check that the cat is cleaning up the gases properly while keeping an eye on fuelling. P0158 means the ECU has seen the voltage from that sensor climb higher than it should, usually above the normal 0.9V or so. That points to either a knackered sensor, a wiring problem, or the engine running rich. Until you sort out which one, the ECU can't trust what that sensor is telling it.
ⓘ Information only. This page provides general educational information about fault code P0158. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0158 mean?
P0158 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: O2 Sensor Circuit High Voltage (Bank 2 Sensor 2).
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on, frequently the only thing you'll actually notice
- • A drop in fuel economy, often in the region of 5 to 15%
- • Lumpy idle or a flat spot when you put your foot down
- • Black smoke from the tailpipe if the cause is a rich mixture
- • Failed emissions readings if you happen to be near MOT time
- • On plenty of cars, nothing changes in how it drives and only the light tips you off
Possible causes
Causes commonly associated with P0158, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. The bank 2 sensor 2 itself has failed or got contaminated, the usual suspect on higher-mileage cars
- 2. Damaged, corroded or shorted wiring in the sensor circuit, common where the loom runs near hot exhaust
- 3. A short to voltage on the signal wire, which pins the reading artificially high
- 4. Exhaust leak ahead of the sensor pulling in outside air and skewing the readings
- 5. Engine running rich from a leaking injector or a failing fuel pressure regulator
- 6. Sensor harness resting against the manifold or downpipe and melting the insulation
- 7. A faulty ECM, which is rare and should be your last guess, not your first
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Plug in a scan tool, pull the codes and have a proper look at the freeze frame data so you can see what the engine was doing when it logged the fault
- 2. Get under the car and inspect the bank 2 sensor 2 connector and loom for corrosion, melted insulation or anything resting on hot metal. Wiring catches a lot of people out here
- 3. Watch the live sensor voltage. If it's sitting stubbornly above 0.9 to 1.0V instead of drifting around mid-range, that confirms the high-voltage condition
- 4. Check for exhaust leaks around the manifold gaskets and pipe joints near the sensor, since false air upsets the reading
- 5. Test the signal circuit with a multimeter for continuity, shorts and open circuits before you condemn the sensor
- 6. If the wiring and circuit all check out, fit a new bank 2 sensor 2, clear the code and take it for a drive to make sure it stays away
Common questions about P0158
Can I just change the sensor myself or clean the old one? +
Cleaning an oxygen sensor is a waste of time, they're not built to be serviced and once the element is contaminated or shot it stays shot. Replacing it is fair game if you've got an oxygen sensor socket and the bank 2 sensor isn't tucked somewhere stupid. A decent post-cat sensor runs roughly £30 to £90 depending on the car. Just don't throw a sensor at it before you've ruled out the wiring and a rich mixture, because if the real fault is a leaking injector or a chafed wire, the new sensor will read high too and you've spent money for nothing.
If I clear the code will it stay gone? +
Only if you've actually fixed what set it. Wipe it with a scanner and the ECM will relog P0158 within a drive cycle or two if the sensor is still reading high or the wiring is still faulty. The code is the symptom, not the disease. Clear it after the repair to confirm the job's done, not as a fix on its own.
What's the harm in leaving it and carrying on driving? +
You can usually keep driving short trips without drama, but it's not something to ignore for months. If the cause is a rich mixture, you'll be burning more fuel and eventually cooking the catalytic converter, which turns a £60 sensor job into a several-hundred-pound cat replacement. There's also the MOT to think about. The code won't fail you on its own, but if the warning light is lit when you roll up for the test, that can go against you, and a rich-running engine may fail on emissions anyway.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →