P0139
PowertrainOxygen Sensor Circuit Slow Response (Bank1, Sensor2)
Most of the time this is a tired rear oxygen sensor that's slowed down with age and exhaust soot, common once a car's past 90,000 miles. The sensor sits after the cat on bank 1, and the ECU watches how quickly its voltage swings when conditions change. When that response gets sluggish and lags behind what the ECU expects, it logs P0139. The sensor still works, it's just gone lazy.
ⓘ Information only. This page provides general educational information about fault code P0139. 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 P0139 mean?
P0139 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Oxygen Sensor Circuit Slow Response (Bank1, Sensor2).
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, frequently with no change you can actually feel in how the car drives
- • A small dip in fuel economy, usually only obvious if you track your MPG on longer runs
- • Occasional hesitation when you get back on the throttle after lifting off
- • Slightly lumpy idle from cold on some engines, settling once warm
- • Emissions failure at MOT if the cat monitoring has been thrown off
Possible causes
Causes commonly associated with P0139, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Aged or soot-contaminated sensor, the usual culprit on higher-mileage cars. Heat cycles and exhaust deposits slow the element down over years
- 2. Corroded or damaged wiring and connectors in the sensor loop, often from road salt and water getting into the plug under the car
- 3. Exhaust leak ahead of or near the rear sensor letting fresh air into the gas stream, which dulls the readings and makes a healthy sensor look slow
- 4. Failed sensor heater so the element takes too long to reach operating temperature, especially noticeable in cold weather
- 5. High resistance in the sensor's ground or signal path, often a single poor pin in the connector
- 6. Restricted or failing cat changing the gas chemistry the rear sensor sees
- 7. Leaking injector on bank 1 throwing off the mixture the sensor has to track
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 codes and watch the rear sensor live data. A good sensor swings reasonably quickly when you change throttle. If it crawls between voltages and lags well behind the front sensor, that's your slow response confirmed
- 2. Get under the car and check the sensor plug and wiring properly. Look for green corrosion on the pins, chafed insulation near the heat shield, and any sign water's been sitting in the connector
- 3. Listen and feel along the exhaust ahead of the rear sensor for leaks with the engine running. A small leak before sensor 2 mimics this code exactly
- 4. Measure the heater circuit resistance cold and compare to the manual figure, typically a few ohms. An open or out-of-spec heater means slow warm-up and a slow signal
- 5. Check resistance through the signal and ground wires back to the ECU connector to rule out a high-resistance fault before you condemn the sensor
- 6. If wiring, exhaust and heater all check out, the sensor itself is almost certainly worn out and due for replacement
Common questions about P0139
Will my car fail the MOT with a P0139 stored? +
The stored code on its own isn't an automatic fail, but if the engine warning light is on when the tester plugs in or eyeballs the dash, that's a fail on its own under current MOT rules. On top of that, a slow rear sensor can upset the cat monitoring and tip you into an emissions failure on the gas test. Fix the cause, clear the light, and drive a few cycles so it stays off before you book the test.
What's the damage to put it right at a garage? +
If it's just the sensor, a quality part is roughly £30 to £90 and an independent will usually have it fitted for £100 to £180 all in. A main dealer charging £130 an hour plus a genuine sensor can easily double that. Wiring repairs run similar to a sensor swap once you add diagnostic time. If it turns out to be the cat behind it, you're into the high hundreds, so it pays to confirm the actual cause first.
How do I work out whether it's the sensor or something else on my car? +
Start with the live data. A lazy sensor shows slow voltage swings while the front sensor is darting about normally. Before you buy a sensor, prod the connector and wiring under the car, because corrosion there is cheap to fix and gives the same symptom. Then have a quick listen for an exhaust leak near the sensor with the engine idling. If the wiring's clean, no leak, and the heater reads in spec, the sensor's the one at fault.
Can I replace or clean the sensor myself? +
You can replace it yourself if you've got the car safely up in the air and an oxygen sensor socket, which is about £15 and saves you rounding off the hex. Cleaning oxygen sensors doesn't really work. People try brake cleaner or a wire brush, but a slow sensor has aged internally and a once-over won't bring the response time back. A new sensor is cheap enough that it's not worth chasing. Just make sure you get the correct part for your model rather than a generic universal one.
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 →