P0138: the rear oxygen sensor signal is reading too high
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 the electrical signal from bank 1's rear sensor is reading higher than the range the computer will accept. With this signal out of range, the engine control unit falls back on a default value instead.
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
P0138 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.