Draft guidance: awaiting Claude spot-check, qualified-mechanic review and named-editor approval. Do not use a fault code alone to replace parts.
P0138: Oxygen sensor circuit high voltage, Bank 1 Sensor 2
The downstream Bank 1 sensor signal is staying above the computer's calibrated high limit.
What P0138 means
The downstream Bank 1 sensor signal is staying above the computer's calibrated high limit. This may come from a biased sensor or a signal wire shorted to voltage, but it can also reflect genuinely rich running.
Can I still drive?
A vehicle that runs normally with a steady warning can generally make a short, gentle trip for diagnosis. Treat raw-fuel smell, black smoke, misfire or suspected catalytic-converter overheating as urgent and stop safely, because sustained over-fuelling can cause expensive damage.
Symptoms
- Engine-management light, often as the only symptom
- Increased fuel use
- Rough running, poor performance or stalling if the engine is genuinely rich
- Strong or rich exhaust smell
Common causes
- Internally faulty downstream sensor
- Heat-damaged signal wire shorted to a supply
- Moisture, corrosion or poor terminal contact at the connector
- Excess fuel pressure or leaking injector
- Other fuel-control fault; control-module failure is uncommon
Safe first checks
- Scan for rich-mixture, misfire, heater and catalyst codes before clearing them. Inspect the cold downstream harness where it passes the exhaust and check the connector for moisture or corrosion. Warm live data and fuel trims can help separate a real rich condition from an electrical bias, but back-probing must follow the correct diagram.
Typical UK fix cost
About £90–£450 for diagnosis and a common repair. A confirmed sensor replacement is often £110–£266; injector, fuel-pressure or substantial wiring faults may exceed this broad range.
Related models
This is a generic OBD-II code used across many makes and models. The exact test thresholds, component layout and repair route vary by engine and manufacturer, so check vehicle-specific technical information before ordering parts.
Related fault codes
- P0136: Oxygen sensor circuit malfunction, Bank 1 Sensor 2
- P0137: Oxygen sensor circuit low voltage, Bank 1 Sensor 2
- P0140: Oxygen sensor circuit no activity detected, Bank 1 Sensor 2
- P0141: Oxygen sensor heater circuit malfunction, Bank 1 Sensor 2
Sources
A diagnostic trouble code identifies a system or circuit to investigate. It does not prove that a named part has failed. Preserve freeze-frame data and seek a vehicle-specific diagnosis before clearing the code.