P0138: O2 Sensor Circuit High Voltage — Bank 1 Sensor 2
What to do
Usually no driveability impact. If fuel economy has also worsened, the rich condition is real and upstream fuelling deserves the attention rather than this sensor.
What this code means
The downstream bank 1 sensor is reporting persistently high voltage. A short to supply is one explanation; a genuinely rich exhaust reaching the rear sensor is another, and that points upstream at fuelling rather than at the sensor. Worth noting that a failed converter can also produce a rear signal that mimics the front one, which is what the catalyst monitor is designed to detect.
Likely causes
Start here
- Faulty oxygen sensor · easy
- Wiring shorted to voltage · moderate
Less common
- Leaking fuel injector · moderate
- Excessive fuel pressure · moderate
- Corroded or loose connector · easy
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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