Worn, contaminated or failed upstream oxygen sensor on Bank 1; Exhaust leak upstream of or near the sensor drawing in air and giving a false lean reading; Vacuum leak or unmetered air entering the intake causing a genuinely lean mixture; Damaged, chafed or shorted sensor wiring, or a corroded or loose connector; Low fuel pressure or a restricted injector or fuel delivery fault causing a lean mixture; Faulty mass airflow sensor under-reading airflow; Fault in the engine control unit sensor circuit or its heater supply.

P0131

O2 Sensor Circuit Low Voltage Bank 1 Sensor 1.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
1: AreaFuel and air metering
31: FaultThe specific fault in that area

What it means

P0131 means the upstream oxygen sensor (Bank 1, Sensor 1) is reporting a voltage that stays too low for too long. The control unit saw the signal sitting below the expected switching range, usually under about 0.1 to 0.2 volts, when it should be swinging up and down as the mixture changes. The sensor may be faulty, or something else may be making the engine run lean or the signal circuit read low.

Symptoms

Engine warning light on
Poor fuel economy
Rough idle or hesitation
Engine running lean, possibly with misfire under load
Failed emissions test
Slow or sluggish response to the throttle

Possible causes

Worn, contaminated or failed upstream oxygen sensor on Bank 1likely
Exhaust leak upstream of or near the sensor drawing in air and giving a false lean readinglikely
Vacuum leak or unmetered air entering the intake causing a genuinely lean mixturelikely
Damaged, chafed or shorted sensor wiring, or a corroded or loose connectorpossible
Low fuel pressure or a restricted injector or fuel delivery fault causing a lean mixturepossible
Faulty mass airflow sensor under-reading airflowpossible
Fault in the engine control unit sensor circuit or its heater supplyrare

How to diagnose

Read the freeze frame data and note engine load, temperature, speed and fuel trim values when the code set, and check for other stored codes
Inspect the sensor wiring and connector for chafing, heat damage, water ingress, corrosion and poor pin tension
Check for exhaust leaks around the manifold, flange joints and the pipe before the sensor
Look for vacuum leaks: inspect hoses, intake joints and the brake servo hose, and review the long-term fuel trims for a lean correction
Watch the live sensor voltage and fuel trims at idle and with a snap throttle; a healthy sensor should swing across its range and respond quickly
Check the sensor heater circuit supply, earth and resistance against the manufacturer specification for your model
Check fuel pressure and the mass airflow sensor readings against the specification for your engine
Check the signal and reference wiring for continuity and for shorts to earth between the sensor and the control unit

How to fix

Repair or replace damaged wiring or the connector, and clear any corrosion
Fix any exhaust leak ahead of the sensor, replacing gaskets or damaged sections
Repair vacuum or intake leaks by replacing split hoses, seals or gaskets
Replace the upstream oxygen sensor if it is slow, stuck low or fails its tests
Correct fuel delivery faults such as low pressure or blocked injectors, and replace a faulty mass airflow sensor if confirmed
Clear the codes after repair and road test to confirm the fault does not return

Can I keep driving?

Medium severity

The car can usually be driven for a short time, but a lean mixture can cause poor running and, over time, higher exhaust temperatures and damage to the catalytic converter. Get it diagnosed soon, and avoid heavy loads or long journeys if it is running badly.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.