P0139: O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)

What does P0139 mean?

The oxygen sensor downstream of the bank 1 catalyst changes its reading too slowly during the monitor test. Rear sensors normally change more slowly than front sensors, so diagnosis depends on the manufacturer test rather than a simple visual comparison.

Generic OBD-II definition: O2 Sensor Circuit Slow Response (Bank 1 Sensor 2). Confirm applicability in your vehicle service information.

Common causes

• Aged or contaminated rear sensor
• Exhaust leak near the sensor
• Damaged wiring or heater operation problem

Symptoms

• Check engine light
• Often no noticeable drivability change
• Emissions monitor may fail

Diagnostic focus

A steady rear signal during normal cruise can be normal catalyst behavior. P0139 means the defined response test failed; it is not enough to see that the rear trace switches less than the front.

Can I keep driving with P0139?

Severity: Low With a steady light and normal running, arrange diagnosis soon. Stop for a flashing light, severe misfire, overheating, or a glowing catalyst; do not assume those symptoms are only a sensor fault.

If the check engine light is flashing or the car runs badly, stop driving and have it checked.

Typical fixes

• Inspect rear-sensor wiring and exhaust sealing
• Check heater codes before response testing
• Test downstream response with the prescribed scan-tool procedure

Sources & review

• OBD-Codes.com: P0139
• TroubleCodes.net: generic code definitions

Definitions and diagnostic background checked 2026-11-06. Generic guidance, not a substitute for the service procedure for your exact year, engine, and transmission.