P0138 — O2 Sensor High Voltage — Bank 1, Sensor 2

What P0138 means

The downstream (post-catalyst) oxygen sensor on Bank 1 is reading a persistently high voltage (rich, near 1V). On a healthy system the rear sensor sits relatively steady in the mid-range (roughly 0.4–0.7V) with little switching; a stuck-high reading usually means a contaminated or failed sensor, a connector/wiring issue, or a genuinely rich condition.

Most likely causes (in order)

1. Failed downstream O2 sensor internally shorted high ($30–80)
2. Engine running rich — excess unburned fuel passing through the catalyst
3. Coolant or oil burning in combustion sending rich exhaust past the catalyst
4. Wiring short to voltage on the signal circuit

Symptoms you might notice

• Check engine light on
• Possible rich smell from exhaust
• No driveability symptoms from this code alone

What to check first

Compare upstream (Sensor 1) and downstream (Sensor 2) O2 readings live. If upstream is also rich and long-term fuel trims are negative, the engine itself is running rich — fix the rich condition first before replacing the downstream sensor. If upstream looks normal and only the downstream is stuck high, the downstream sensor has failed.

Repair cost & difficulty

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