Emission control
P0131 — O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1)
The upstream oxygen sensor on bank 1 (before the catalytic converter) reports a voltage that stays too low, meaning it sees a lean mixture or has a circuit fault. The ECU can no longer fine-tune fuel properly.
Common causes
- Aged or failed oxygen sensor
- Exhaust leak near the sensor
- Vacuum or intake leak causing a lean mixture
- Damaged sensor wiring or connector
- Low fuel pressure or clogged injectors
Symptoms
- Higher fuel consumption
- Rough idle or hesitation
- Check engine light
- Failed emissions test
How to check
- Check for exhaust and intake leaks
- Inspect the sensor connector and wiring
- Watch live sensor voltage: it should switch between about 0.1 and 0.9 V
- Check fuel trims; replace the sensor if it stays low while the mixture is correct
Can you keep driving?
Usually drivable for a while, but fuel economy and emissions get worse. Repair it soon.
Related codes
Frequently asked questions
- What does P0131 mean?
- The upstream oxygen sensor on bank 1 reads constantly low voltage, as if the mixture were lean.
- Is P0131 always a bad oxygen sensor?
- No. A real lean condition from a leak or fuel problem can cause the same code.
General reference information. It does not replace a diagnosis on your own vehicle. The same code can have different causes depending on make, model year and engine; check the manufacturer's documentation or ask a technician with a scan tool.