P0138 O2 Sensor Circuit High Voltage Bank 1 Sensor 2

P0138 indicates high voltage on the bank 1 sensor 2 oxygen-sensor circuit. This downstream sensor signal may be stuck rich, shorted, or affected by exhaust and wiring faults.

Code details

Generic OBD-II · P0

System: Fuel / Air
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not treat P0138 as a catalyst diagnosis by itself; verify the sensor circuit and engine mixture first.

Symptoms of P0138

Check-engine light on, sometimes with rough or uneven running.
Hesitation, surging, or a noticeable change in fuel economy.
Occasionally hard starting or a smell of unburnt fuel.

Common causes of P0138

A failing or contaminated sensor (MAF, MAP, oxygen or air-fuel sensor).
Intake or vacuum leaks, or an exhaust leak near an upstream sensor.
Wiring, connector, or fuel-delivery problems that skew the reading.

Useful checks before replacing parts

Compare downstream sensor voltage to upstream sensor activity and fuel trims.
Inspect the downstream sensor harness for melting, short-to-voltage, water intrusion, or connector damage.
Check for rich-running conditions or catalyst-monitor faults that can influence downstream readings.

Related OBD-II codes

These nearby fuel / air entries can help compare wording and pin down the fault area.