P0139: Downstream O2 sensor slow response (Bank 1 Sensor 2)
You can keep driving. Get it checked when convenient.
Can I drive with P0139?
Yes, you can keep driving. The post-cat sensor is reacting slowly to emissions swings; driving is unaffected.
What does P0139 mean?
P0139 stands for O2 Sensor Circuit Slow Response (Bank 1, Sensor 2).
Symptoms you may notice
- Check engine light on with no noticeable change in how the car drives
- Vehicle fails state emissions smog test
- Normal engine power and shifting
How much does it cost to fix?
Repair costs for code P0139 depend on the specific underlying root cause, ranging from simple DIY checks to component replacement. The table below lists the most common causes and rough price ranges in USD:
- Old or carbon-contaminated downstream O2 sensor
- Small exhaust leak near the catalyst flange
- Contaminated sensor tip from oil or fuel additives
- Catalytic converter starting to degrade
What to do next
- Inspect exhaust system around the catalytic converter for soot streaks indicating leaks.
- Replace the downstream oxygen sensor.
- Clear the fault code and re-test.
Last updated: October 2026
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General guidance only, not a professional diagnosis. If the check-engine light is flashing, or the car is running rough, overheating or losing power, stop driving and get help.
About this code
Code P0139 means the downstream oxygen sensor is taking too long to switch states during computer diagnostic test routines. Since downstream sensors rarely switch rapidly when the catalytic converter is functioning, this code typically points to an old, sluggish sensor element. Drive without worry and replace at your convenience.