P0138
O2 Sensor Circuit High Voltage Bank 1 Sensor 2
Fuel & air metering (O2 sensor)
Generic OBD-II
Medium
What it means
P0138 means the downstream oxygen sensor on bank 1 (after the catalytic converter) is reporting an abnormally high voltage — a rich signal. Because this sensor checks the converter rather than controlling fuel, the engine drives normally. It usually points to a failed sensor, a wiring short to voltage, or a genuine rich condition.
Medium
Can I keep driving? Yes — this is the downstream (post-catalyst) oxygen sensor, so the car drives normally; it mainly affects emissions monitoring, but get it checked so a real fuel or exhaust problem isn't hidden.
Common symptoms
What you'll notice
- Check-engine light on (often the only symptom)
- Slightly worse fuel economy in some cases
- Fails an emissions test
Common causes
Most likely first
- Failed downstream oxygen sensorLikely
- Wiring or connector shorted to voltage, or a poor connectionCommon
- Exhaust leak near the sensorCommon
- Engine actually running rich (fuel-trim or injector issue)Less often
How to diagnose & fix
Work through it in order
- Read freeze-frame and live O2 data to see if the voltage is truly stuck high.
- Inspect the sensor's wiring and connector for damage, corrosion, or a short to power.
- Check for exhaust leaks near the sensor.
- Rule out a rich-running condition before condemning the sensor.
- Replace the downstream O2 sensor if it tests faulty. Clear and re-test.
FAQ
Quick answers
Is it safe to drive with P0138?+
Yes. Bank 1 Sensor 2 is after the catalytic converter and doesn't control fueling, so drivability is normal. Still, fix it so the emissions system can monitor properly.
Can I just clear P0138?+
It returns unless the sensor, wiring, or rich condition is fixed. Clearing only helps confirm whether the fault is current or was a one-time glitch.
Related codes
Often seen with P0138
Read & clear P0138 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX