Faulty downstream oxygen sensor (bank 1 sensor 2) sending a high voltage signal; Short to voltage in the sensor signal circuit or a chafed wire touching a power source; Poor sensor ground connection causing a reference voltage offset; Contaminated sensor from oil or coolant entering the exhaust stream; Exhaust leak upstream of the sensor allowing outside air to skew readings; Engine running genuinely rich due to leaking fuel injectors or high fuel pressure; Failing catalytic converter that is no longer storing oxygen properly

DTC codes / P01xx · Fuel and air metering / P0138

P0138O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)

Q: What does code P0138 mean?

O2 Sensor Circuit High Voltage (Bank 1 Sensor 2). P0138 means the downstream oxygen sensor on bank 1 is reporting a voltage that stays higher than the engine computer expects for the current operating conditions. The computer monitors the sensor's signal circuit and sets this code when the voltage remains elevated, indicating a rich exhaust reading or a fault in the sensor circuit itself.

P0138 means the downstream oxygen sensor on bank 1 is reporting a voltage that stays higher than the engine computer expects for the current operating conditions. The computer monitors the sensor's signal circuit and sets this code when the voltage remains elevated, indicating a rich exhaust reading or a fault in the sensor circuit itself.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering

Symptoms you may notice

Illuminated check engine light, often with no other noticeable symptoms
A slight decrease in fuel economy if the sensor is biased rich
Rough idle or hesitation in some cases when the false rich signal affects fuel trim
A sulfur or rotten egg smell from the exhaust if the engine is actually running rich
Failed emissions test due to the stored trouble code

Common causes, most likely first

Faulty downstream oxygen sensor (bank 1 sensor 2) sending a high voltage signal
Short to voltage in the sensor signal circuit or a chafed wire touching a power source
Poor sensor ground connection causing a reference voltage offset
Contaminated sensor from oil or coolant entering the exhaust stream
Exhaust leak upstream of the sensor allowing outside air to skew readings
Engine running genuinely rich due to leaking fuel injectors or high fuel pressure
Failing catalytic converter that is no longer storing oxygen properly

How to diagnose and fix it

Connect a scan tool and record freeze-frame data to see when the code set and what the sensor voltage was
Inspect the sensor wiring and connector for damage, corrosion, or signs of a short to voltage
Check the sensor ground circuit for excessive resistance using a multimeter
Measure the sensor signal voltage with a voltmeter while the engine is running to confirm it stays high
Perform a propane enrichment test or check fuel trims to determine if the engine is actually running rich
Replace the downstream oxygen sensor if voltage testing confirms it is stuck high
Clear the code and verify the repair with a test drive that includes closed-loop operation

Why a high voltage does not always mean a bad sensor

A short to voltage in the signal wire is a common cause that many technicians miss because they replace the sensor first without checking the circuit.
A contaminated sensor from coolant or oil burning in the exhaust can read high without any driveability complaint, so inspect for leaks before condemning the sensor.
An exhaust leak between the engine and the sensor can pull in outside air and cause a false lean reading, but a leak after the sensor can trap rich exhaust and set a high voltage code.
The downstream sensor's only job is to monitor catalytic converter efficiency, so a high voltage code does not always indicate a failed converter; the sensor circuit must be ruled out first.
Some vehicles use a titania sensor that changes resistance rather than generating voltage, and testing procedures differ, so always confirm the sensor type before probing.

Can I keep driving with P0138?

You can usually continue driving with P0138 because the engine computer defaults to a safe fuel strategy, but avoid heavy loads and long trips until the fault is diagnosed. Prolonged driving with a rich condition can damage the catalytic converter and increase emissions.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.