What P0138 means
The downstream oxygen sensor is reading high voltage, often a rich condition or a shorted signal wire.
Common causes
- Rich running condition
- Coolant or silicone contamination of the sensor
- Signal wire shorted to voltage
How it is usually fixed
- 1Check fuel trims for a rich bias
- 2Inspect wiring
- 3Replace the downstream sensor
Other common codes
- P0011 — Camshaft Position A – Timing Over-Advanced (Bank 1)
- P0030 — HO2S Heater Control Circuit (Bank 1, Sensor 1)
- P0101 — Mass Air Flow Circuit Range/Performance
- P0113 — Intake Air Temperature Sensor Circuit High
- P0118 — Engine Coolant Temperature Sensor Circuit High
- P0131 — O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1)
Code information is general guidance, not a diagnosis for a specific vehicle. Confirm with live data and manufacturer test procedures before replacing parts.