P0138
PowertrainO2 Oxygen Sensor Circuit High Voltage (Bank1, Sensor2)
The bank 1 sensor 2 (rear) oxygen sensor is reporting voltage stuck high. Either the engine is running rich for a sustained period (rich fuel mixture passing through the cat), or the sensor itself has failed in a high-voltage state.
ⓘ Information only. This page provides general educational information about fault code P0138. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0138 mean?
P0138 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: O2 Oxygen Sensor Circuit High Voltage (Bank1, Sensor2).
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light, often the only symptom
- • Possibly worse fuel economy if a genuine rich condition exists
- • Drivability typically normal
- • Smell of unburned petrol from the exhaust on cars with a real rich condition
Possible causes
Causes commonly associated with P0138, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Genuine rich condition (failed front sensor, leaking fuel injector, fuel pressure too high)
- 2. Failed rear sensor stuck high (internal failure mode where the signal pegs at maximum)
- 3. Sensor wiring shorted to a higher voltage source
- 4. Damaged cat allowing unburned fuel through to the rear sensor
- 5. Coolant temperature sensor reading too cold causing the ECU to run rich (the front sensor reports lean, fuel is added, the rear sensor sees the rich mixture passing through)
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Read live fuel trim data first; long-term trim well negative confirms genuine rich condition that's the real cause
- 2. Check the front oxygen sensor signal, if the front is bouncing rich-lean normally and only the rear is stuck high, the rear sensor is the suspect
- 3. Inspect for related codes (P0172, P0420) that point upstream
- 4. Check coolant temp sensor reading at full operating temperature, should be around 90°C
- 5. Inspect sensor wiring for shorts to power
Common questions about P0138
Could this be a damaged catalytic converter? +
Possibly. A cat that's broken down internally (substrate cracked or melted) doesn't catalyse properly, and unburned fuel reaches the rear sensor causing it to read high. Check whether you have P0420 alongside P0138, that combination is more strongly indicative of a cat issue than a sensor issue.
How can I tell if it's the sensor or a real rich condition? +
Look at fuel trims. Long-term trim sitting well into negative figures (-15% or worse) confirms a genuine rich condition, the sensor is reporting truthfully and you need to find the cause. Healthy trims with stuck rear-sensor reading point at the rear sensor itself.
Will replacing the rear sensor fix it? +
If the underlying cause is a genuinely rich engine, no, you've replaced the messenger but the message is still real. New sensor will read the same. Fix the rich condition first; if the rear sensor still misbehaves after that, then replace it. Don't skip the diagnostic step.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →