P0131 O2 Sensor Circuit Low Voltage Bank 1 Sensor 1

P0131 indicates low voltage on the bank 1 sensor 1 oxygen-sensor circuit. The upstream signal is lower than expected and may be circuit-related or mixture-related.

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 assume low voltage proves a bad sensor; a real lean condition or signal-circuit problem can create the same code.

Symptoms of P0131

Check-engine light.
Possible rich/lean drivability issues.
Poorer fuel economy.

Common causes of P0131

A failed upstream oxygen sensor.
An exhaust leak near the sensor.
A short-to-ground or wiring fault in the signal circuit.
A genuinely lean exhaust condition.

Useful checks before replacing parts

Check for exhaust leaks, lean fuel trims, low fuel pressure, unmetered air, and oxygen-sensor signal wiring faults.
Compare bank 1 sensor 1 response with commanded enrichment and with other available oxygen or air-fuel sensor data.
Inspect connector pins, sensor ground, harness routing, and contamination that can bias the signal low.

Related OBD-II codes

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