Trouble codes › P0139
P0139 O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)
Oxygen sensors
P0139 means bank 1's rear oxygen sensor is responding too slowly, usually because it's old.
The rear oxygen sensor on bank 1 is responding too slowly when the engine computer changes the mixture (often tested during deceleration). It's usually an aging sensor.
Safe to drive for now. It affects the catalytic converter check and emissions readiness.
Common causes
- An aging, slow rear oxygen sensor
- Contamination from oil or coolant
- An exhaust leak near the sensor
Symptoms
- Check-engine light
- Usually nothing else
What to check first
- Check the sensor's age and look for exhaust leaks.
- Watch the sensor's response during deceleration with a scan tool.
- Replace with the correct sensor for your vehicle.
What does P0139 mean on your vehicle?
The likely cause depends on your engine. iRepair researches your exact year, make, model and engine (service bulletins, recalls and owner reports) and tells you what to check first. Your first 3 questions are free.
Ask iRepair about P0139No scanner? Most auto parts stores read check-engine codes for free.
Related codes
General guidance for P0139 on most vehicles. Some makers use the same code a little differently; confirm in the service information for your vehicle, put safety first, and see a professional when in doubt.