P0131: O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1)

What does P0131 mean?

The bank 1 upstream oxygen-sensor signal remains below the expected range. On conventional narrowband sensors this can look lean, but an exhaust leak or an electrical fault can create the same reading.

Generic OBD-II definition: O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1). Confirm applicability in your vehicle service information.

Common causes

• Exhaust leak near the upstream sensor
• Signal circuit shorted to ground
• Faulty or contaminated oxygen sensor

Symptoms

• Check engine light
• Possible hesitation or poor fuel economy
• Sometimes no noticeable driving symptom

Diagnostic focus

Do not diagnose a lean engine from one low-voltage value alone. If P0171 is also present, investigate actual mixture problems; a ground short can hold the voltage low even when the mixture changes.

Can I keep driving with P0131?

Severity: Medium With a steady light and normal running, arrange diagnosis soon. Stop for a flashing light, severe misfire, overheating, or a glowing catalyst; do not assume those symptoms are only a sensor fault.

If the check engine light is flashing or the car runs badly, stop driving and have it checked.

Typical fixes

• Check fuel trims and inspect for exhaust leaks
• Test the sensor signal and return wiring
• Confirm the sensor responds under a controlled mixture test before replacement

Sources & review

• OBD-Codes.com: P0131
• TroubleCodes.net: generic code definitions

Definitions and diagnostic background checked 2026-11-06. Generic guidance, not a substitute for the service procedure for your exact year, engine, and transmission.