P0139O2 Sensor Circuit Slow Response Bank 1 Sensor 2
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- 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.
- What you may notice
- Check engine light on (usually after two drive cycles)
- Catalyst monitor readiness not set
- P0420 may follow
- Likely causes
- LikelySensor aged or contaminated (oil ash, silicone, additives)
- PossibleExhaust leak ahead of the sensor
- PossibleSensor heater weak, sensor too cold
- RareHigh-resistance ground or connector
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Easy · Often possible to do yourself.Typical cost: €60 – €250
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.