P0131 Front oxygen sensor voltage too low

By: Diagno editorial team · Updated: 2026-10-15

P0131 means: Front oxygen sensor voltage too low. The front oxygen sensor measures oxygen in the exhaust before the catalytic converter. OK to drive, but have it checked soon. The repair usually costs about €60–250 in Estonia. The most common cause: sensor is old or faulty.

What it means

The front oxygen sensor measures oxygen in the exhaust before the catalytic converter. The engine computer uses it to set the fuel mixture. The sensor reports a lean mixture all the time. Often the wire is damaged or there is an air leak.

What could be wrong

1. 1Sensor is old or faulty
2. 2Wire or connector is damaged
3. 3Exhaust leak in front of the sensor
4. 4The mixture really is off, for example an air leak

What you can check yourself

Check the sensor wire and connector. Listen for an exhaust leak anywhere before the catalytic converter. You can check the simple things yourself, but the repair is best done in a workshop.

How we fix it

1. We check the sensor reading with a diagnostic tool
2. We check for exhaust leaks and the mixture
3. We replace the sensor if it is faulty

Standard SAE description: O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1)