OBD-II code · Powertrain (generic)
P0138 O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
The downstream oxygen sensor signal is fixed at a high voltage, indicating either a continuously rich downstream reading (less common, since post-cat exhaust should be more stable) or a short to supply voltage in the signal circuit.
Is it safe to drive with P0138?
Usually safe to drive in the short term. Get it diagnosed when convenient, sooner if the car drives differently.
Common causes
- Most likelySignal wire shorted to supply voltage
- PossibleDownstream sensor failed
- Less likelyConnector pins corroded - high contact resistance
Symptoms
- Check engine light on
- Increased fuel consumption
- Possible rough idle or hesitation
- Failed emissions test
Repair at a glance
- Difficulty
- Easy
- DIY friendly
- Often, with basic tools
- Typical labour
- 0.3–1.5 h
- Emissions test
- Likely to fail
- After the fix
- A drive cycle is needed before the light stays off
- Parts involved
- Oxygen Sensor Downstream, Oxygen Sensor Wiring
Related codes
- P0132O2 Sensor Circuit High Voltage (Bank 1, Sensor 1)
- P0152O2 Sensor Circuit High Voltage (Bank 2, Sensor 1)
- P0135O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)
- P0136O2 Sensor Circuit Malfunction (Bank 1, Sensor 2)
- P0137O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2)
- P0139O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
- P013AO2 Sensor Slow Response - Rich to Lean Bank 1 Sensor 2
- P013BO2 Sensor Slow Response - Lean to Rich Bank 1 Sensor 2
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.