P0139
O2 Sensor Circuit Slow Response Bank 1 Sensor 2
Fuel & air metering (O2 sensor)
Generic OBD-II
Medium
What it means
P0139 means the downstream oxygen sensor on bank 1 (behind the catalytic converter) is switching too slowly for the ECM's self-test. It's usually a worn or contaminated sensor, sometimes wiring or an exhaust leak. Because this sensor is used mainly to monitor the catalytic converter rather than control fuel, the engine runs normally — the main effects are the warning light and emissions readiness.
Medium
Can I keep driving? Safe to drive — this downstream oxygen-sensor fault won't affect how the engine runs, but it keeps the light on and can fail an emissions test.
Common symptoms
What you'll notice
- Check-engine light on
- Slightly reduced fuel economy
- Fails an emissions test
- Otherwise runs normally
Common causes
Most likely first
- Aged or contaminated downstream (bank 1, sensor 2) oxygen sensor responding too slowlyLikely
- Corroded or loose sensor connector or damaged wiringCommon
- Exhaust leak near the sensorCommon
- Failed engine control module (rare)Less often
How to diagnose & fix
Work through it in order
- Graph the bank 1 sensor 2 signal live — a sluggish, lazy switching pattern confirms slow response.
- Inspect the sensor connector and wiring for corrosion or damage.
- Check for exhaust leaks around and upstream of the sensor.
- Replace the downstream O2 sensor if it responds slowly with good wiring. Clear and re-test.
FAQ
Quick answers
Is it safe to drive with P0139?+
Yes. The rear O2 sensor mainly monitors the catalytic converter, so drivability is essentially unaffected — the practical impact is the warning light and a possible emissions-test failure.
Can I just clear P0139?+
You can, but the ECM will re-test the sensor's response and reset the code if it's still sluggish.
Related codes
Often seen with P0139
Read & clear P0139 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX