Check Engine Code
P0138
O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
Severity: medium
DIY difficulty: 2/5
Typical cost: $40–$2000
What P0138 means
Downstream sensor reads stuck-rich. Often points to a failing catalyst (not storing oxygen) or coolant intrusion contaminating the sensor.
Symptoms
- CEL
- Often paired with P0420
Likely causes (most common first)
- Failing catalyst
- Coolant intrusion (head gasket failure)
- Failed downstream O2 sensor
- Short to power
Can I keep driving?
Medium — drivable but fix soon
Typical repair cost
DIY fixes can start near $40 (parts only). Shop repairs for P0138 typically run up to $2000 depending on the underlying cause and vehicle. Start with the most common cause — Failing catalyst — before throwing parts at it.
Related codes
Confirm P0138 yourself in 60 seconds
AXLY.pro is a free iPhone app that pairs with any Bluetooth OBD2 adapter to read P0138 and every other stored code, plus live engine data. No subscription.