P0155
PowertrainO2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)
Most people notice nothing different about how the car drives, and the warning light is the first sign anything's wrong. You might see the fuel economy slip a touch on short journeys around town, where the engine never gets fully warm. Behind the scenes, the heater element inside the front oxygen sensor on Bank 2 isn't doing its job. That heater is meant to bring the sensor up to operating temperature quickly so the ECU can start trimming the fuel mixture properly from cold, and when the circuit faults the ECU flags P0155.
ⓘ Information only. This page provides general educational information about fault code P0155. 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 P0155 mean?
P0155 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1).
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, usually with no obvious change in how the car drives
- • Fuel economy dropping a bit, most noticeable on cold mornings and short stop-start trips
- • Slightly lumpy idle in the first few minutes after a cold start, then it settles
- • On some cars a richer fuel smell or a puff of darker smoke from the exhaust on start-up
- • Emissions readiness monitors refusing to complete, which matters if an MOT is due
- • Sluggish throttle response while the engine is still warming through
Possible causes
Causes commonly associated with P0155, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Heater element inside the sensor itself has failed, which is the usual answer on higher-mileage cars. Heater elements just wear out with age and heat cycles
- 2. Damaged or corroded wiring and connectors in the heater circuit. Bank 2 sensor 1 sits in a hot, exposed spot near the manifold and the wiring takes a battering
- 3. Blown fuse feeding the heater circuit. Check this before you condemn anything pricey
- 4. A poor earth connection adding resistance and starving the heater of proper voltage
- 5. Water finding its way into a connector, common on cars that wade through floods or get jet-washed too keenly
- 6. Open or short circuit somewhere in the heater wiring run
- 7. ECM fault or a calibration that's behind on updates, though this is rare and the last thing to suspect
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. Read all stored and pending codes first. If P0155 turns up alongside its Bank 1 twin P0135, suspect a shared fuse or earth rather than two dead sensors at once
- 2. Pull the fuse for the oxygen sensor heater circuit and check it. A blown fuse is the cheapest fix you'll find and people skip straight past it
- 3. Get under the car and eyeball the sensor, its loom, and the connector for melted insulation, corrosion, bent pins, or water sitting in the plug
- 4. With the ignition on, back-probe the connector and check you're getting roughly 12V on the heater supply. No voltage points at the wiring or fuse, not the sensor
- 5. Measure resistance across the heater terminals on the sensor side. You're looking for somewhere around 2 to 15 ohms depending on the make. An open reading means the element is gone
- 6. Clean up and tighten any earth connections you find looking tatty before you spend money on parts
Common questions about P0155
What actually goes wrong if I just keep driving with this on? +
Day to day, not much. The car will run, but until the heater works the sensor takes far longer to start reporting on cold starts, so the ECU runs on a default fuel map for longer than it should. That means a bit more fuel burned and more raw fuel through the exhaust on short trips. Over months that extra unburnt fuel can shorten the life of the catalytic converter, which is a far bigger bill than a sensor. It's a slow burn, not an emergency.
How quickly do I need to sort this out? +
You're not going to be stranded and it won't drop into limp mode over P0155 alone, so there's no need to panic. Treat it as a job for the next couple of weeks rather than the next car park. The exception is if your MOT is due, because the warning light being on at test time can cost you the pass, and the emissions readiness monitors may not complete in time. Get it booked in before the test, not the day before.
Is it the sensor itself or just the wiring? +
Both are common, so don't just throw a sensor at it. The quick checks decide it. Measure resistance across the heater terminals: if it's open or way outside the 2 to 15 ohm ballpark, the element inside the sensor has failed and the sensor needs replacing. If the resistance reads fine but there's no 12V reaching it, the fault is in the fuse, the wiring, the earth, or a corroded connector. A genuine sensor runs roughly £40 to £120, while a wiring or fuse repair can be next to nothing if you spot it early.
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 →