P0138: O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
The oxygen sensor after the catalytic converter on bank 1 is reading high voltage for too long, which looks like rich exhaust.
What P0138 means
According to RepairPal, P0138 sets when the downstream oxygen sensor voltage stays above about 450 millivolts for more than roughly 20 seconds, with limits varying by maker. High voltage means little oxygen, which reads as rich exhaust. The rear sensor watches how well the catalytic converter is storing oxygen, so the computer expects its voltage to move a little but not stay pinned high. If it does, either the engine is truly running rich, the sensor has failed, or the signal wire is shorted to a voltage source. A common electrical failure is the signal wire touching the heater power wire in a melted harness, which drives the reading high. Shops look at fuel trims and the upstream sensor to rule out a rich engine, then unplug the downstream sensor and watch the signal. If it stays high with the sensor unplugged, the wiring is at fault. If it drops, the sensor itself is the problem.
Common symptoms
- Check engine light on
- Often no driveability change
- Possible fuel smell from the exhaust
- Lower gas mileage if the engine is truly rich
- Failed emissions test
Most common causes
- Failed downstream oxygen sensor
- Signal wire shorted to the heater or voltage
- Engine running rich from a leaking injector or high fuel pressure
- Fuel or oil contaminating the sensor
- Engine computer software issue
What to check yourself first
Check whether rich codes like P0172 are also present. Inspect the downstream sensor wiring for melted spots where the heater and signal wires could touch. Note any fuel smell from the tailpipe.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Replace oxygen sensor | $440-$523 | The common fix when the engine mixture is normal. |
| Electrical diagnosis to find a wiring or connector fault | $122-$179 | For a signal wire shorted to the heater circuit inside the harness. |
| Fuel pressure test | $60-$88 | If the engine itself seems rich. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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Related codes
- P0137 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)
- P0136 O2 Sensor Circuit (Bank 1 Sensor 2)
- P0172 System Too Rich (Bank 1)
- P0420 Catalyst System Efficiency Below Threshold (Bank 1)
- P0141 O2 Sensor Heater Circuit (Bank 1 Sensor 2)