P0138: O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)

P0138 means the downstream oxygen sensor on Bank 1 (post-catalyst, Sensor 2) is reporting a voltage that is too high for too long, above roughly 1.0V. A healthy downstream sensor reads around 0.6–0.8V steady. Persistent high voltage typically indicates a rich exhaust condition, a short to voltage in the wiring, or a contaminated sensor.

P0138 is a emissions-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P0138 and illuminates the check engine light.

If your vehicle has triggered P0138, you may notice one or more of these symptoms:

• Check engine light illuminated
• Increased fuel consumption
• Black smoke or strong fuel smell from exhaust in severe cases
• Possible failed emissions test
• May appear with P0172 or P0175 if the engine is genuinely running rich

Here are the most common causes of P0138, ranked by how likely they are to be the culprit:

Generally yes, for short-term driving. Safe to drive short term. A high downstream signal usually points to a rich condition reaching the sensor, a shorted sensor, or a failing catalytic converter. Long-term rich running can clog the converter, so do not ignore this for more than a few weeks.

Follow these steps to pinpoint the root cause of P0138:

1. Read freeze frame data and check for companion codes (P0172, P0175, P0420) that point to a real rich condition or converter problem
2. Use a scan tool to view downstream sensor voltage. A reading stuck above 0.9V at idle indicates a rich condition or a faulty sensor
3. Check long-term fuel trims. If LTFT is significantly negative (under -10%), the engine is genuinely running rich
4. Inspect the Bank 1 Sensor 2 signal wire for shorts to voltage, especially near heat shields and the battery positive cable
5. Substitute a known-good sensor. If high voltage persists, the issue is fuel mixture or wiring, not the sensor

Repair costs for P0138 vary depending on the root cause and your vehicle. Here are typical estimates:

P0138 behaves differently depending on your vehicle. Select your car below for model-specific causes, known issues, and adjusted cost estimates:

These codes are often seen alongside P0138 or indicate related issues: