What it means
The upstream oxygen sensor signal is fixed at a low voltage, indicating the ECU sees a continuously lean exhaust on bank 1 — or the sensor signal path is shorted to ground.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The upstream oxygen sensor, Bank 1 Sensor 1, measures oxygen in the exhaust before the catalytic converter so the control module can adjust fueling. P0131 sets when its signal voltage stays below the expected range. This can reflect a genuinely lean exhaust or a signal circuit shorted to ground.
Common causes
Most likely first.
- Exhaust leak upstream of the sensor
- Signal wire shorted to ground
- Genuine lean running (vacuum leak, low fuel pressure)
Symptoms
- Check engine light on
- Increased fuel consumption
- Possible rough idle or hesitation
- Failed emissions test
How to diagnose it
- Save freeze-frame data and review live Bank 1 Sensor 1 data with fuel trims to see whether the low signal occurs with evidence of a lean mixture.
- Inspect the exhaust for leaks upstream of Bank 1 Sensor 1, since outside air entering there can make its reading appear lean.
- Check the Bank 1 Sensor 1 connector and harness for heat damage, chafing, corrosion, or a signal wire touching ground.
- If live data supports a lean condition, check for intake leaks and verify fuel delivery using the manufacturer's specification.
- Test the sensor signal circuit for a short to ground and verify sensor operation before replacing the upstream sensor.
Common fixes
- Repair an exhaust leak ahead of Bank 1 Sensor 1.
- Repair a signal wire shorted to ground or other damaged wiring.
- Correct an intake leak or fuel delivery fault confirmed by testing.
- Replace Bank 1 Sensor 1 if its signal remains low with a sound circuit and no genuine lean condition.
Can I keep driving?
Limit driving if the engine runs rough or hesitates, and avoid long trips until the lean reading is diagnosed. A persistent mixture fault can raise fuel use and emissions and may damage the catalytic converter.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.