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.

Real-world scan data

Seen in

304

scans in the past year

Found in

0.1%

of scans

0.1%

of scans that found faults

Appears alongside · share of P0139 scans

P0420in 17.8% of scans

Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.

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.

Read & clear P0139 yourself

Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.

▶ Get it on Google Play

✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX