Fault codeP0131

MODERATE

O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1)

What this code means

O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1).

“Bank 1” is the side of the engine that holds cylinder number 1; “Bank 2” only exists on V and flat engines. An in-line engine has a single bank.

“Sensor 1” is the oxygen sensor upstream of the catalytic converter, the one that regulates the mixture; “Sensor 2” sits downstream and monitors the converter.

P0131 is a generic code: it means the same thing on every make. P0131 belongs to family P01xx — Fuel and air metering. Its page explains how to read a code and groups all its codes by part.

Oxygen sensor (lambda sensor)

The oxygen sensor is screwed into the exhaust and measures how much oxygen is left after combustion. The upstream sensor (sensor 1) lets the control unit keep the mixture right on the ideal ratio, correcting it many times a second; the downstream sensor (sensor 2), after the catalytic converter, checks that the converter is doing its job.

Symptoms

Higher consumption, rough idle, hesitation, sometimes a smell of petrol. The upstream oxygen sensor stays stuck at low voltage — a permanent “lean” signal — and the control unit enriches wrongly, or treats the sensor as mute.

Common causes

Ageing or fouled oxygen sensor (oil, additives, coolant)
Intake air leak or exhaust leak before the sensor, diluting the gases
Fuel pressure too low, making the mixture genuinely lean
Sensor loom shorted to ground, or corroded connector
Failed sensor heater, leaving the sensor cold for too long

How to diagnose it

Look at the sensor signal in live data, engine warm: it should swing several times a second between 0.1 and 0.9 V. Frozen below 0.2 V, either the mixture is genuinely lean — strongly positive fuel trims, see P0171 — or the sensor no longer responds.

Tell the two apart by briefly enriching, a squirt of cleaner into the intake: if the voltage rises, the sensor works and you need to find the leak; if it does not move, replace the sensor.

Watch it live in Klaro

With an OBD2 adapter plugged in, these live readings show the part at work, when your car reports them:

O2 sensor voltage, bank 1 sensor 1
O2 sensor voltage, bank 1 sensor 2
Short term fuel trim bank 1
Long term fuel trim bank 1
Commanded equivalence ratio (lambda)

What to do

Have it checked this week. The car remains usable, but the fault will not clear itself and will fail the roadworthiness test.

Clearing a code switches the light off but repairs nothing: if the fault remains, the code comes back. Clearing a fault code yourself →

Repair cost

Oxygen sensor: €50 to €200 for the part, €20 to €80 to fit if it is not seized. An exhaust leak costs €50 to €300 to repair depending on where it is.

How to read this code

PPowertrain
0standardised code
1Fuel and air metering
31P0131