P0157
PowertrainO2 Sensor Circuit Low Voltage (Bank 2 Sensor 2)
The ECU watches the voltage coming back from the rear oxygen sensor on Bank 2, the one sitting after the catalytic converter, and it expects that signal to hover and drift gently around 0.45V. When the voltage sits stuck down low and won't lift the way it should, the ECU flags P0157. For you that usually points to a dying sensor or a wiring problem on that bank, and on a V6 or V8 it's the side of the engine furthest from cylinder number one.
ⓘ Information only. This page provides general educational information about fault code P0157. 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 P0157 mean?
P0157 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: 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:
- • Check engine light on, often the only thing you'll actually notice
- • Slightly worse fuel economy, more obvious on longer motorway runs than around town
- • Idle and drivability usually feel completely normal, which catches a lot of people out
- • If the ECU starts trusting the duff signal it can nudge the mixture rich, so you might smell a richer exhaust
- • Emissions creeping up over time, which only shows itself at MOT
Possible causes
Causes commonly associated with P0157, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Failed rear oxygen sensor on Bank 2, far and away the usual answer once a car is past 90,000 miles and the sensor heater and element have had enough
- 2. Damaged or corroded wiring to the sensor, common where the loom runs near the hot exhaust and the insulation has cooked
- 3. Poor earth at the sensor or chassis point, a dirty ground gives a low or lazy reading
- 4. Exhaust leak just ahead of the sensor pulling fresh air into the stream and pushing the voltage down
- 5. Connector full of moisture or road salt, very common on UK cars in winter
- 6. Low fuel pressure or a genuine lean condition on that bank, less likely but worth ruling out
- 7. Oil or coolant contamination on the sensor tip, usually a sign of a bigger engine problem
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 live data and watch the Bank 2 Sensor 2 voltage at idle and on light throttle. A healthy rear sensor lazily floats around 0.45V. If it's pinned low and dead, you've found your fault area
- 2. Unplug the connector and check both halves for green corrosion, bent pins, or water sitting inside. Clean it up and clear the code before you spend a penny on parts
- 3. Follow the loom from the sensor back, looking for chafed insulation or melted sections where it's touched the exhaust
- 4. Back-probe the sensor signal and earth wires with a multimeter. Confirm a clean ground and that the ECU is supplying its reference, often around 5V with the ignition on
- 5. Listen and feel for an exhaust leak in front of the sensor, a small blow there will fake this code all day long
- 6. If wiring, earths and the exhaust all check out, the sensor itself is the culprit and it's time for a new one
Common questions about P0157
Should I buy a budget sensor off eBay or pay for a proper one? +
Stick with a known brand. Bosch, Denso or NGK depending on what the car came with, and you'll pay roughly £40 to £120 for the part. The £15 no-name sensors are a false economy, they often switch sluggishly or fail the heater circuit within months and you'll be back here with the same code. On anything 2018 onwards the ECU is fussier about non-OE sensors too, so a cheap one can throw the light straight back up. Match the part to your engine and don't gamble on the cheapest listing.
Can I keep driving with P0157 showing? +
For everyday use, yes, it's usually fine for a while. This is the post-cat sensor, so it monitors emissions rather than running the fuelling minute to minute, and the car won't drop into limp mode over it. The catch is if the low reading is being caused by a real lean condition or an exhaust leak, because that can cook the catalytic converter over time and that's a far bigger bill. Get it looked at within a few weeks rather than ignoring it for months.
Is this going to fail my MOT? +
The code on its own isn't a fail, but if the engine warning light is glowing on the dash when the tester looks, that's an automatic fail under current MOT rules. So if you've fixed the cause, drive it enough miles to clear the light before you book the test. If the underlying problem has knocked your tailpipe emissions out of spec, the emissions check can fail you separately as well.
What's it likely to cost to sort? +
A rear oxygen sensor replaced at a decent independent garage usually lands around £100 to £200 all in, sensor and labour. A main dealer will charge more, often pushing toward £250 to £350 once you add their hourly rate. If it turns out to be wiring or a connector rather than the sensor, a competent garage can repair that for less than a new part, so make sure they actually diagnose it rather than just throwing a sensor at it.
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 →