What does P0138 oxygen sensor high voltage bank 1 sensor 2 mean?
Short answer: The downstream oxygen sensor on bank 1 is reading higher voltage than expected for too long, which can indicate a rich condition, a shorted sensor or a wiring fault.
The downstream sensor normally reads a fairly steady voltage after a working catalyst. If it stays above the expected maximum, the computer sets P0138. A genuinely rich exhaust, perhaps from a leaking injector or high fuel pressure, can cause it, but sensor and wiring faults are common too.
Check fuel trims and upstream sensor data first. If the engine is running normally and trims are close to zero, the downstream sensor is the suspect. Inspect its wiring for melted insulation against the exhaust, which can short the signal to the heater circuit.
With the key on and sensor unplugged, check the signal wire for voltage that should not be there. A sensor contaminated by coolant or oil can also read high. Replace the sensor only after ruling out wiring problems, route the new harness away from hot exhaust components to prevent a repeat failure, and let the exhaust cool first to avoid burns.
Tools used for this job
General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.