Check Engine Code
P0138
O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
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.
Symptoms that match P0138
If you got here from a search query, one of these might describe what your car is actually doing:
Recommended parts & tools for P0138
Our buyer’s guides cover the tools and parts that pair with AXLY:
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