P0139 Oxygen sensor circuit slow response (bank 1, sensor 2)
What P0139 means
The engine computer tests the downstream oxygen sensor on bank 1 by briefly changing the mixture and timing how quickly the sensor reacts. The sensor responded too slowly, so the computer can no longer rely on it to monitor the catalytic converter.
P0139 is a generic SAE J2012 code: it means the same on every make. The exact cause still depends on the car, so read the freeze-frame data before replacing parts.
At a glance
- Severity
- Medium
- Typical repair cost
- 80–350 €
- Workshop time
- 0.3–1.5 h
- Repair difficulty
- Easy
- DIY possible
- Yes
- Check engine light
- Yes
- Affects emissions test
- Yes
Parts and labour at an independent workshop in Europe. Main dealers charge more; the cause decides the bill, not the code.
Symptoms
- Check engine light on
- Usually no noticeable change in how the car drives
- Catalyst readiness monitor not completing
- Possible failed emissions test at inspection
Common causes
- Ageing downstream oxygen sensor
- Sensor contaminated by oil, coolant or fuel additives
- Exhaust leak near the sensor
- Wiring or connector with high resistance
Can I keep driving with P0139?
You can keep driving, but have it checked within one to two weeks. The engine is rarely affected, but the catalyst check cannot run and the car may fail its emissions inspection.
What to check first
- Check for exhaust leaks near the downstream sensor.
- Inspect the connector and wiring for corrosion or damage.
- Watch the sensor response in live data during a sharp throttle blip and a lift-off; it should react within a second or so.
- Replace the sensor if it remains slow.
Related codes
- P0138 Oxygen sensor circuit high voltage (bank 1, sensor 2)
- P0137 Oxygen sensor circuit low voltage (bank 1, sensor 2)
- P0136 Oxygen sensor circuit (bank 1, sensor 2)
- P0135 Oxygen sensor heater circuit (bank 1, sensor 1)
- P0134 Oxygen sensor circuit no activity detected (bank 1, sensor 1)
- P0133 Oxygen sensor slow response (bank 1 sensor 1)
- P0132 Oxygen sensor circuit high voltage (bank 1, sensor 1)
- P0140 Oxygen sensor circuit no activity detected (bank 1, sensor 2)
Data: TORKO fault-code catalogue, built on OBDex (CC0-1.0) and reviewed against workshop practice. Informational only — it does not replace a diagnosis by a mechanic.
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