P0139 code meaning
O2 Sensor Circuit Slow Response Bank 1 Sensor 2
Powertrain system Generic code P01xx codes Check engine light
What P0139 means
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.
P0139 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyEasy, DIY possible
- Typical labor0.3 to 1.5 hours
- Clears after repairAfter a drive cycle
Symptoms of P0139
- Check engine light on (usually after two drive cycles)
- Catalyst monitor readiness not set
- P0420 may follow
Common causes of P0139
- Most likelySensor aged or contaminated (oil ash, silicone, additives)
- PossibleExhaust leak ahead of the sensor
- PossibleSensor heater weak, sensor too cold
- Less likelyHigh-resistance ground or connector
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Oxygen sensor bank1 sensor2Oxygen sensor wiringCatalytic converter
What each part of P0139 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 1 | Area: fuel and air metering. |
| 39 | The exact fault in that area. |
What to do next
- Note P0139 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0139
What does P0139 mean?
P0139 means "O2 Sensor Circuit Slow Response Bank 1 Sensor 2". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with P0139?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P0139 fail an emissions test?
Yes, it can. P0139 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0139 to fix?
Difficulty: easy. A typical repair takes 0.3 to 1.5 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P0139 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.