P0139: the rear oxygen sensor is responding too slowly
A second oxygen sensor sits in the exhaust after the catalytic converter and checks how well the converter is cleaning up the exhaust, rather than fine-tuning the fuel mixture the way the front sensor does. This code means bank 1's rear sensor is taking longer to switch between its readings than the computer expects. A slow-reacting sensor here makes it harder for the computer to judge how well the catalytic converter is working.
You might notice
- The engine warning light comes on
- No other symptom you'd notice while driving, since this isn't the sensor the computer uses to fine-tune fuelling moment to moment
- oxygen sensor
- — a sensor in the exhaust that measures how much oxygen is left after combustion
- catalytic converter
- — the part of the exhaust that cleans up the gases before they leave the car
What the code itself says
P0139 is a powertrain (engine, gearbox and drive system) code, and the 0 after the letter means it is a generic code with a standard meaning.
Grouped here as: Engine running, fuelling and misfires — Air metering, fuelling, ignition, misfire, idle, boost, knock, oxygen sensors and the catalyst, all working together in the running engine. That grouping is ours, to help explain the code. The code's official family is the letter at the start.
This page is generic. Your car isn't.
A code on its own only tells you what the sensor saw, not what's actually wrong — and that changes next to your other codes. Read this one together with the rest of your scan and we'll work out what's really going on.
Stop reading. Start diagnosing.
Two minutes, free, and it'll be about your car rather than every car.