Drive with caution — fix soon.
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.
$150 – $450
Varies by vehicle and root cause.
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.
Replacing the downstream oxygen sensor typically costs $150-$450 including labor. Wiring repairs can change the total depending on the damage.
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.
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.