P0056
PowertrainHO2S Heater Control Circuit (Bank 2 Sensor 2)
Most of the time this is a failed heater element inside the rear oxygen sensor on Bank 2, which is the cheapest part of the sensor to give up. Every modern lambda sensor has a small built-in heater that brings it up to operating temperature fast on a cold start, so it can start reading exhaust gases before the engine itself is properly warm. When the ECU sends current to that heater and the resistance is wrong or the circuit is open, it logs P0056 for the downstream sensor on Bank 2. On a V6 or V8 this is the sensor on the side furthest from cylinder one, after the cat.
ⓘ Information only. This page provides general educational information about fault code P0056. 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 P0056 mean?
P0056 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: HO2S Heater Control Circuit (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, usually with nothing else wrong that you'd actually feel
- • Slightly worse fuel economy on short cold-weather trips, since the sensor takes longer to come online
- • More exhaust emissions during the warm-up minute or two after a cold start
- • The odd bit of rough running or hesitation from cold, though this is uncommon with a rear sensor
- • Plenty of cars show no driving symptoms at all, just the light
Possible causes
Causes commonly associated with P0056, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. The heater element inside the sensor has failed. This is the usual answer, and the heater dies long before the sensing element does
- 2. Wiring to the sensor chafed, melted, or corroded. The rear sensor sits near hot exhaust and the loom takes a beating over the years
- 3. Blown fuse or tired relay feeding the heater circuit. Worth two minutes before you condemn anything else
- 4. Corroded or loose pins in the sensor connector, often from road salt and water sitting in the plug
- 5. Broken or rusted exhaust ground strap upsetting the heater circuit reference
- 6. Faulty ECM driver for the heater. Rare, and the last thing you check, not the 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. Read live data and pull any other codes. If P0058 or a partner P0030-series code is also stored, that points you at wiring or the ECM driver rather than just a dead sensor
- 2. Find the Bank 2 Sensor 2 sensor and inspect the loom and connector properly. Look for melted insulation, green corrosion in the plug, and a rotten ground strap
- 3. Check the heater fuse and relay in the fusebox. A single blown fuse can take out heaters on more than one sensor at once
- 4. Measure heater resistance across the two heater pins at the sensor connector. Most petrol sensors read somewhere around 3 to 15 ohms cold, but check your manual figure. Open circuit or way out of spec means the sensor
- 5. With the sensor unplugged, confirm you have battery voltage on the supply side and a clean ground when the engine's running. No power means the fault is upstream in the wiring
- 6. If the supply is good and the heater resistance is open, fit a new sensor and clear the code, then drive a couple of cold cycles to confirm it stays gone
Common questions about P0056
Can I sort this myself, or is it a garage job? +
You can have a go. Pop the rear Bank 2 sensor connector apart first and check for corrosion and a blown heater fuse, because that's a free fix if you're lucky. If the heater element has failed, a new sensor and an oxygen sensor socket (about £15) gets it done on the driveway, as long as you can physically reach the sensor and the old one isn't seized solid in the exhaust. Diesel cars and tucked-away V-engine banks can be a fight, so factor that in before you start.
If I just wipe the code, will it stay off? +
If the heater element is actually dead or the wiring is broken, no. The ECU tests that circuit on the next cold start and the light comes straight back, often within a drive cycle or two. The only time clearing it sticks is when the real trigger was a one-off, like a connector you've since reseated or a fuse you've replaced. Clearing without fixing just hides it until your next journey.
What's the harm in leaving it for a bit? +
Mechanically the car will keep running and won't strand you. What you lose is clean running during warm-up, so fuel economy on short trips dips and emissions go up while the engine's cold. The bigger nuisance is that with the light already on, this code can mask a more serious one that crops up later, and a lit MIL means an automatic fail at MOT. Sort it within a few weeks rather than ignoring it for months.
How quickly do I need to deal with it? +
It's not an emergency. There's no immediate damage risk to the engine or the cat from a faulty rear sensor heater, unlike a misfire or a rich-running fault. The urgency is really driven by your MOT. If a test is coming up, fix it first, because the warning light alone will fail you. A new sensor is roughly £100 to £300 fitted at an independent garage, less if you do the labour yourself.
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 →