P0139: O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)
The rear oxygen sensor on bank 1 reacts more slowly than expected.
Normal risk
Safe to drive. Have it checked at the next opportunity.
Short answer · Can I keep driving with P0139?
Yes. The rear sensor mainly checks the catalytic converter, so driving is not affected. Have it fixed before an emissions test.
What does P0139 mean?
Code P0139 means the oxygen sensor after the catalytic converter on bank 1 switches too slowly during the control unit’s test, for example when fuel is cut on deceleration. An ageing sensor or an exhaust leak are common causes.
What the car does when the code is set
The check engine light comes on. The engine still controls the mixture, but less precisely, so fuel economy may drop slightly and emissions rise. Most drivers notice no change in how the car drives.
How the system works
The front oxygen sensor measures the oxygen left in the exhaust after combustion. A narrowband sensor switches between about 0.1 volts (lean) and 0.9 volts (rich), and the engine keeps the mixture swinging around the ideal point. Wideband sensors give a continuous signal instead. The faster the sensor responds, the more precisely the engine can control the mixture. The engine control unit measures how long the sensor takes to switch, or to follow a deliberate change in mixture, and stores a code when the response is slower than its limit.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Aged oxygen sensorSensors slow down with age and contamination.
- Exhaust leak near the sensorOutside air distorts the reading.
- ContaminationOil, coolant or silicone residues coat the sensor tip.
- Wiring faultDamaged wiring or connector problems.
How it is diagnosed
- Check for related codesFix misfire, fuel-trim and heater codes first.
- Watch the sensor signalOn a warm engine at about 2,500 rpm, a narrowband front sensor should switch quickly between about 0.1 and 0.9 volts. Slow, lazy switching confirms the code.
- Inspect for leaks and contaminationLook for exhaust leaks and inspect the sensor tip for white, black or oily deposits.
- Replace with the correct sensorUse the sensor type specified for your engine and clear the code afterwards.
Questions about P0139
Can P0139 cause a catalyst code?
A slow rear sensor can make the catalyst test unreliable. Fix the sensor first, then see whether P0420 returns.
Can a lazy oxygen sensor cause a catalyst code?
Yes. A slow front sensor degrades mixture control, and the test of the converter can then fail. Fixing the sensor first saves money.
How long do oxygen sensors last?
Many manufacturers specify no fixed interval, but sensors often slow down after about 150,000 km. Contamination shortens their life.