P0138 – O2 Sensor Circuit High Voltage (Bank 1 Sensor 2) means the downstream oxygen sensor on Bank 1 is reporting a voltage that is **consistently higher than expected**. This sensor monitors catalytic converter performance and helps the ECU verify that emissions are being properly reduced. When the voltage stays too high, it usually indicates a rich exhaust condition, wiring short, or a failing sensor. This guide explains what P0138 means, the symptoms, causes, diagnosis steps, and the best repair solutions.
What Does P0138 Mean?
P0138 is a generic powertrain code indicating the downstream O2 sensor on Bank 1 is stuck at a voltage above the normal operating range. Downstream sensors typically read between 0.1–0.9 volts, but with this fault, the sensor may be stuck above ~0.9V for too long.
The ECU detects this as an abnormal condition, often caused by a rich mixture, sensor contamination, wiring short-to-power, or internal O2 sensor failure.
Quick Reference
- OBD-II Family: P-Code (Powertrain)
- Scope: Generic
- System: Fuel & Emissions / Oxygen Sensor
- Difficulty Level: Easy–Moderate
- Estimated Repair Cost: €60–€300
- Last Updated: 4 December 2025
Real-World Example / Field Notes
A sedan came in with P0138 and a strong fuel smell. The downstream sensor voltage was pinned at 0.98V. The issue was a leaking fuel injector causing a rich condition, which also affected upstream sensor trims. After replacing the injector and clearing the code, normal downstream voltage returned.
Another common case is a wiring short-to-power. One SUV had the O2 sensor signal wire melted onto the heater circuit wire, forcing the signal above 1.0V continuously. Repairing the harness resolved P0138 without replacing the sensor.
Symptoms of P0138 – O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
- Check engine light: MIL illuminated with P0138 stored.
- Poor fuel economy: Rich conditions may increase consumption.
- Strong exhaust odor: Often indicates excessive fuel.
- Failed emissions test: Catalyst may not operate efficiently.
- Rough idle or hesitation: Rare but possible.
Common Causes of P0138
Most Common Causes
- Short-to-power in O2 sensor signal wire
- Faulty downstream O2 sensor (Bank 1 Sensor 2)
- Rich air/fuel mixture
- Fuel injector stuck open or leaking
- Contaminated or soaked catalytic converter
Less Common Causes
- Faulty fuel pressure regulator causing rich conditions
- Wiring harness melting near exhaust components
- ECU internal interpretation fault (rare)
- Incorrect aftermarket sensor installation
- Exhaust system restriction or backpressure
Diagnosis: Step-By-Step Guide
P0138 diagnosis focuses on verifying sensor voltage, checking for rich conditions, and identifying wiring shorts.
Tools You’ll Need
OBD-II scan tool with live data, multimeter, fuel pressure tester (optional), wiring diagrams, and basic tools.
- Check live O2 sensor voltage: If it stays above ~0.9V, the circuit is stuck high.
- Inspect wiring and connectors: Look for melted insulation, corrosion, or shorts-to-power.
- Check for rich running: Look at fuel trims, injector pulse width, and upstream O2 readings.
- Inspect for fuel leaks: A leaking injector or high fuel pressure can cause rich exhaust.
- Check catalytic converter health: A fuel-soaked cat may produce high downstream voltage.
- Perform a wiggle test: Move the harness and monitor voltage for dropouts.
- Check fuel pressure: High pressure can cause excessive fuel delivery.
- Test O2 sensor heater circuit: A malfunctioning heater can skew sensor output.
- Confirm sensor response: Apply throttle variations and watch for voltage changes.
- Replace the downstream O2 sensor if sensor behavior remains abnormal.
Pro Tip: If the upstream O2 sensor is also reading rich, diagnose fueling issues before replacing the downstream sensor — otherwise you may fix the symptom, not the cause.
Possible Fixes & Repair Costs
- Repair melted or shorted wiring: €50–€150
- Replace downstream O2 sensor: €80–€180
- Repair fuel injector leaks: €100–€300+
- Replace fuel pressure regulator: €120–€250
- Repair or replace catalytic converter: €250–€900+
Can I Still Drive With P0138?
Yes, the vehicle will usually run, but a rich condition can damage the catalytic converter and worsen fuel economy. Driving long-term with P0138 is not recommended because excess fuel in the exhaust can overheat and harm the catalyst.
Need wiring diagrams and factory-style repair steps?
Powertrain faults often require exact wiring diagrams, connector pinouts, and guided test steps. A repair manual can help you confirm the cause before replacing parts.
Related Sensor Circuit Codes
Compare nearby sensor circuit trouble codes with similar definitions, fault patterns, and diagnostic paths.
- P0132 – O2 Sensor Circuit High Voltage Bank 1 Sensor 1
- P0699 – Sensor Reference Voltage “C” Circuit High
- P0653 – Sensor Reference Voltage “B” Circuit High
- P0643 – Sensor Reference Voltage “A” Circuit High
- P0193 – Fuel Rail Pressure Sensor Circuit High
- P0179 – Fuel Composition Sensor Circuit High
Key Takeaways
- P0138 means the downstream O2 sensor is stuck at a high voltage on Bank 1.
- Most common causes include sensor failure, wiring shorts, or a rich fuel mixture.
- Ignoring the code can damage the catalytic converter.
- Proper diagnosis requires checking both wiring and fuel system operation.
FAQ
Is P0138 the upstream or downstream O2 sensor?
Sensor 2 always refers to the downstream (after the catalytic converter) O2 sensor.
Can a rich mixture trigger P0138?
Yes. Excess fuel in the exhaust increases downstream sensor voltage.
Can I drive with this code?
You can, but long-term driving risks catalyst damage and poor fuel economy.
Does P0138 always mean the sensor is bad?
No. Wiring shorts or fueling issues can also cause high voltage.
Can a failing catalytic converter cause P0138?
Yes, especially if the converter becomes fuel-soaked or restricted.