P0138 — O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
Drive with caution — fix soon.
What P0138 means
P0138 means the rear oxygen sensor on Bank 1, located after the catalytic converter, is reading a continuously high voltage. This often points to a failing sensor or its wiring, but a stuck-rich fuel condition or a shorted signal wire can also trigger it. Because this sensor monitors converter performance, accurate readings matter for passing emissions.
Common symptoms
- Check engine light on
- Often no change in how the car drives
- Possible slight drop in fuel economy
- Failed emissions test
- May appear alongside a catalyst code like P0420
Common causes
- Failed downstream oxygen sensor (Bank 1, Sensor 2)
- Short to voltage in the sensor signal wiring
- Corroded or damaged sensor connector
- Rich fuel condition from another fault
- Coolant or oil contamination of the sensor
- Wiring chafed against the exhaust or chassis
Typical repair cost
$150 – $450
Varies by vehicle and root cause.
Frequently asked questions
Can I drive with a P0138 code?
Yes, short-term driving is generally safe and the car usually feels normal. Get it diagnosed soon so it doesn't mask a converter problem or cause an emissions failure.
How much does it cost to fix P0138?
Replacing the downstream oxygen sensor typically costs $150-$450 including labor. Wiring repairs can change the total depending on the damage.
Is P0138 serious?
It's moderate. The car stays drivable, but the rear sensor's job is to monitor emissions and the converter, so an inaccurate reading should be fixed.
Does the downstream sensor affect engine performance?
Not much directly. The rear oxygen sensor mainly watches the catalytic converter, so a P0138 usually impacts emissions monitoring more than how the engine runs.