P0131: O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1)
The upstream oxygen sensor on bank 1 is reading low voltage for too long, which looks like a constantly lean exhaust.
What P0131 means
RepairPal explains that P0131 sets when the upstream oxygen sensor voltage stays below about 400 millivolts for more than about 20 seconds, or when an air-fuel ratio sensor stays lean biased too long, with exact limits varying by maker. Low voltage on a standard oxygen sensor means lots of oxygen in the exhaust, which is what a lean mixture looks like. So the question is whether the engine is really lean or the sensor is lying. An exhaust leak ahead of the sensor can suck in outside air and make it read lean even when the mixture is fine. Vacuum leaks and weak fuel pressure make the engine truly lean. A sensor that is failing or a signal wire shorted to ground can produce the same reading. Shops check fuel trims, propane enrich the engine to see whether the sensor responds, and inspect the exhaust and wiring. If the sensor will not go rich when fuel is added, it gets replaced.
Common symptoms
- Check engine light on
- Rough idle or hesitation
- Lower fuel economy
- Possible lean codes like P0171
- Failed emissions test
Most common causes
- Failing upstream oxygen or air-fuel ratio sensor
- Exhaust leak ahead of the sensor
- Vacuum leak making the engine truly lean
- Low fuel pressure
- Signal wire shorted to ground
What to check yourself first
Look at fuel trims with a scan tool. If long term fuel trim is high positive, the engine may really be lean and the sensor is just reporting it. Check for exhaust leaks at the manifold and vacuum leaks at hoses.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Replace oxygen sensor | $440-$523 | When the sensor fails a response test with a good fuel mixture. |
| Smoke test for vacuum or exhaust leaks | $61-$90 | Smoke testing finds vacuum and exhaust leaks that cause real lean readings. |
| Fuel pressure test | $60-$88 | If fuel trims are high and fuel pressure is suspect. |
| Electrical diagnosis to find a wiring or connector fault | $122-$179 | For a shorted signal wire. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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Related codes
- P0130 O2 Sensor Circuit (Bank 1 Sensor 1)
- P0133 O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)
- P0171 System Too Lean (Bank 1)
- P0137 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)
- P0151 O2 Sensor Circuit Low Voltage (Bank 2 Sensor 1)