P0138 – O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)

What does P0138 mean?

The downstream oxygen sensor (Bank 1, Sensor 2) is reporting persistently high voltage, indicating a rich exhaust condition or a sensor circuit fault such as a short to voltage.

What causes P0138?

Listed from most to least common:

1. 1Failed downstream oxygen sensor
2. 2Short to voltage in the sensor signal wire
3. 3Genuine rich running condition
4. 4Contaminated sensor from oil or coolant consumption
5. 5Corroded connector causing signal error
6. 6Leaking fuel injector

Code breakdown

Common symptoms

• Check engine light on
• Normal drivability in most cases
• Possible fuel smell if genuinely running rich
• Failed emissions test

How to diagnose

1. 1Check for accompanying rich codes such as P0172 — if present, address the rich condition first.
2. 2Graph downstream O2 voltage. A reading pinned near or above 0.9V constantly suggests either a real rich condition or a circuit fault.
3. 3Disconnect the sensor. If the voltage stays high with the sensor unplugged, the fault is in the wiring, not the sensor.
4. 4Inspect the connector for corrosion or contamination.
5. 5Check for oil or coolant consumption that could be poisoning the sensor.

Usually no drivability impact, but it prevents emissions monitors from completing and can indicate a genuine rich condition worth investigating.

Common fixes and estimated costs

Costs are rough US estimates including parts and labor. Actual pricing varies significantly by vehicle, region, and shop rates.

Good to know

The unplug test is quick and decisive: if the high voltage persists with the sensor disconnected, you have a wiring problem and a new sensor will not fix it.

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