P0139: O2 Sensor Circuit Slow Response Bank 1 Sensor 2

What does P0139 mean?

The oxygen sensor after the catalytic converter on bank 1 (sensor 2) is a switching sensor (about 0.1–0.9 V) used for converter monitoring and fine mixture trim. P0139 means the sensor reacts too slowly to mixture changes. Subaru measures it around fuel cut-off: rich to lean from 500 to 300 mV must take no more than 0.51 s, lean to rich from 300 to 500 mV no more than 2 s; two drive cycles set the MIL. CARB requires such a test because a sluggish downstream sensor could let a worn converter pass. Since about 2009–2011 GM and Ford report this as direction-specific codes P013A–P013F instead; P0139 remains in use on older, Asian and Jaguar Land Rover systems. When it fails, the control module stops the downstream trim and cannot evaluate the converter; there is no limp mode.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Check engine light on (usually after two drive cycles)
• Catalyst monitor readiness not set
• P0420 may follow

Common causes

• High Sensor aged or contaminated (oil ash, silicone, additives)
• Medium Exhaust leak ahead of the sensor
• Medium Sensor heater weak, sensor too cold
• Low High-resistance ground or connector

Likelihood is a general estimate across vehicles; your make and model may differ.