Trouble code · Powertrain · Generic (all makes)

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

The oxygen sensor after the catalytic converter on bank 1 is reporting a voltage that stays too high – a reading that looks permanently 'rich'.

Safe to drive?
Usually OK to drive
Get it checked soon
Scanner you need
Any OBD2 scanner
Even a basic code reader shows it
NHTSA complaints quoting it
10
Vehicle complaints, 2015–2025

Common causes

  • Failed downstream oxygen sensor
  • Wiring shorted to voltage or a damaged connector
  • Engine genuinely running rich

Symptoms

  • Usually just the check engine light
  • Failed emissions test

Can I drive with it?

Usually safe to drive. Get it fixed before your next emissions test.

Tip: If the engine also has a rich code (P0172), fix that first – the sensor may be telling the truth.

Which scanner reads it?

Any OBD2 scanner reads it. Live data showing the downstream sensor's voltage helps tell a stuck sensor from a rich engine.

Budget tools like the ANCEL AD310 read it; tools with live data and graphing – like the BlueDriver or OBDLink MX+ – help you find the cause. Not sure which fits you? Try the Find My Scanner quiz.

In the data

How common is it? At New York’s emissions inspections in 2020–21, P0138 was the #20 code found on cars that failed – 0.9% of all the codes found.

Of the vehicle complaints owners filed with NHTSA from 2015 to 2025, 10 quote P0138 – most often from Chevrolet, Toyota, Jeep owners. Complaints lean toward serious problems, so treat this as a sign of how often the code turns up in real reports, not how often cars get it.

Both sources are compared on the Trends page.

General information to help you understand a code – not a diagnosis. Your car’s service information or a qualified mechanic has the final word. See how we research.