P0161
PowertrainOxygen O2 Sensor Circuit Low Voltage (Bank 2, Sensor 2)
Most of the time this comes down to the heater element inside the rear oxygen sensor on bank 2 packing up, or the wiring to it getting cooked by exhaust heat. The ECU feeds that sensor a heater circuit to bring it up to temperature quickly after a cold start, and when it sees the voltage on that circuit drop below what it expects, it logs P0161. Bank 2 is the side of the engine that doesn't have cylinder number one, so this is usually a V6 or V8 thing rather than a four-pot. Sensor 2 means the one sitting after the cat.
ⓘ Information only. This page provides general educational information about fault code P0161. 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 P0161 mean?
P0161 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Oxygen O2 Sensor Circuit Low 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 notice
- • Fuel economy slightly down, mostly in the first minute or so after a cold start while the sensor is still warming itself
- • A bit of rough running or hesitation during warm-up on some engines
- • Higher emissions until the engine is fully up to temperature
- • Car may run marginally rich until warm
- • Plenty of cars show no drivability problem at all beyond the light
Possible causes
Causes commonly associated with P0161, 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 has failed or gone high-resistance, the usual culprit at higher mileage as the element ages
- 2. Wiring to the sensor chafed, melted or corroded from sitting near a hot exhaust and vibrating for years
- 3. Connector at the sensor plug loose or full of green corrosion, very common on cars that see a lot of winter salt
- 4. Blown fuse or duff relay feeding the heater circuit, worth ruling out early because it's cheap
- 5. Bad earth on the heater circuit causing the voltage to read low
- 6. Exhaust leak upsetting the sensor near where it sits
- 7. PCM heater driver itself faulty, rare, leave this as a last resort
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. Pull the live data and confirm it's P0161 and note anything alongside it. A P0141 or P0155 turning up at the same time points more towards wiring or a fuse than the sensor itself
- 2. Get under the car and have a proper look at the bank 2 rear sensor wiring and plug. Brittle insulation, melted sections near the manifold, or a connector packed with corrosion will often answer the question on the spot
- 3. Check the heater fuse and relay against the fuse box diagram. A blown fuse can take out more than one sensor at once, which is a useful clue
- 4. Measure the heater element resistance across the two heater pins with the plug off. You're looking for roughly 5 to 8 ohms depending on the manufacturer. Open circuit or way out of range means the sensor's done
- 5. With the ignition on, confirm you've got around 12V supply and a clean earth at the connector. No power means the fault is upstream in the loom, not the sensor
- 6. If the wiring, fuse and supply all check out clean, fit a new sensor, clear the code and road-test to make sure it stays gone
Common questions about P0161
Will this stop my car passing its MOT? +
The code on its own isn't an MOT item, but if the engine warning light is glowing on the dash when the tester looks at it, that's a fail on the MIL check. There's also a knock-on issue: a stored heater fault can stop the emissions readiness monitors completing, and a tester may refuse the car as 'not ready'. Sort the fault, clear the code, then drive a good 50 to 60 mixed miles before the test so the monitors have time to run.
What's it likely to cost me to put right? +
If it turns out to be a fuse or a corroded plug you've cleaned up, you're looking at a couple of quid or nothing at all. A new rear oxygen sensor is roughly £40 to £120 for a decent aftermarket part, and an independent garage will usually do the lot for £100 to £200 fitted. A main dealer will want OE pricing and their hourly rate, which can take it to £250 plus. Chasing a melted section of loom takes longer and the labour creeps up accordingly.
How do I know whether it's the sensor or just the wiring on my car? +
Two quick checks separate them. First, measure the heater resistance at the sensor pins: if it's open circuit or miles off the 5 to 8 ohm range, the sensor's internal heater has failed and that's your answer. If the resistance is fine, switch the ignition on and check for 12V and a good earth at the plug. No power arriving means the fault is in the fuse, relay or wiring rather than the sensor, so don't waste money on a new sensor until you've found out which it is.
Can I just swap the sensor myself, and should I do both rear sensors while I'm at it? +
Yes, if you can get the car up safely and you've got an oxygen sensor socket, it's a manageable job. A bit of penetrating oil the night before helps, because these thread into the exhaust and can seize solid. As for doing both, only the bank 2 rear sensor has actually failed here, so there's no need to replace bank 1 just because. If the car's well over 100,000 miles and the other rear sensor is the original, some people fit the pair to save going back under there in six months, but that's your call on budget.
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 →