P0138O2 Sensor Circuit High Voltage Bank 1 Sensor 2
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- The oxygen sensor after the catalytic converter on bank 1 (sensor 2) is a switching sensor (about 0.1–0.9 V) used for converter monitoring and fine mixture trim. P0138 means the signal stays above the upper limit. A zirconia sensor can hardly produce more than about 1 V, so readings above 1.05–1.15 V point to an outside voltage such as the heater supply on the signal wire; GM also sees an open circuit here because of its bias voltage. Ford: above 1.1 V for 10 s; GM: above 1050 mV. Subaru again uses a functional test: the minimum voltage never falls below 250 mV within 200 s after fuel cut-off, so the sensor looks permanently rich. Monitored continuously, two trips. When it fails, the control module stops the downstream trim and cannot evaluate the converter; there is no limp mode.
- What you may notice
- Check engine light on (usually after two drive cycles)
- Catalyst monitor readiness not set
- Usually no noticeable change in driving
- Likely causes
- LikelySignal wire shorted to heater supply or battery voltage
- PossibleSensor faulty internally (heater-to-signal short)
- PossibleSignal circuit open or high-resistance (GM bias logic)
- RareGenuinely rich exhaust or oil/coolant contamination
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Easy · Often possible to do yourself.Typical cost: €60 – €250
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.