Fuel & air metering · Powertrain · generic (SAE-defined)
P0131: O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1)
P0131 means the upstream oxygen sensor on bank 1 is reporting a voltage that stays lower than expected, which reads as a constant lean condition or a shorted circuit.
Standard title per SAE J2012 / ISO 15031-6 · Updated 2026-11-07 · By CarWhere Research
How P0131 sets
The powertrain control module (PCM) monitors the bank 1 sensor 1 oxygen (O2) sensor, which sits ahead of the catalytic converter. A conventional sensor produces low voltage when the exhaust is lean and higher voltage when rich. If the signal stays below the expected range for a set time while the engine is warm and in closed loop, the PCM flags P0131. Threshold values and timing are manufacturer-specific.
Common causes of P0131
Roughly most likely first; the actual cause depends on the vehicle and needs testing to confirm.
- Signal wire shorted to ground or chafed against the exhaust
- Failed bank 1 sensor 1 oxygen sensor
- Exhaust leak near the sensor drawing in outside air
- Genuine lean condition from a vacuum leak or low fuel pressure
- Contaminated sensor from coolant, oil or silicone
- Poor connector contact or corrosion
P0131 symptoms
- Check engine light on
- Possible hesitation or rough running if fueling is affected
- Reduced fuel economy
- Lean fuel trim codes may appear alongside it
How P0131 is diagnosed
- Read all codes and freeze frame; if P0171 is also present, look for a real lean condition first.
- Inspect the sensor wiring for contact with the exhaust and for melted insulation.
- Check for exhaust leaks ahead of the sensor that could let in extra oxygen.
- Watch live data: briefly add propane or snap the throttle; a healthy sensor should swing toward a rich reading.
- If the sensor never reads rich, unplug it and check the signal circuit for a short to ground.
- Review short and long term fuel trims to tell a sensor fault from a true lean mixture.
Typical fixes for P0131
- Repair shorted or heat-damaged sensor wiring
- Replace the bank 1 sensor 1 oxygen sensor
- Seal exhaust leaks upstream of the sensor
- Correct vacuum leaks or fuel delivery problems causing a real lean mixture
Replace parts only after testing confirms the cause — a code names the circuit or condition that failed a check, not necessarily the faulty part. We don’t publish per-repair prices; for what households actually spend on upkeep, see car repair costs.
Manufacturer notes
On vehicles that use a wideband air-fuel ratio sensor in this position, low-circuit codes refer to current or voltage behavior that differs from a switching sensor; definitions can vary by manufacturer; check factory service information.
Related codes & warning lights
P0131 FAQ
What does code P0131 mean?
P0131 means the upstream oxygen sensor on bank 1 is reporting a voltage that stays lower than expected, which reads as a constant lean condition or a shorted circuit. The standard definition is "O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1)."
Can I drive with a P0131 code?
Generally yes for short trips, but fuel control may be compromised. Diagnose it within a few days so the engine does not run in a poorly controlled mixture for long.
Does P0131 mean my engine is actually running lean?
Not necessarily. The sensor may be reporting lean because it has failed or the signal wire is grounded, so compare fuel trims and test the sensor response before assuming the mixture is the problem.
Can an exhaust leak cause P0131?
Yes. A leak upstream of the sensor can pull outside air into the exhaust stream, making the sensor read leaner than the engine actually is.
Keep researching
Engine guides by brand (known issues, specs)
More fuel & air metering codes
Sources
- SAE J2012 — Diagnostic Trouble Code Definitions (standard code titles)
- ISO 15031-6 — Road vehicles: communication between vehicle and external equipment for emissions-related diagnostics, Part 6: Diagnostic trouble code definitions
- U.S. EPA — Vehicle Emissions Inspection and Maintenance (I/M) programs
- U.S. EPA, 40 CFR 86.1806-17 — Federal onboard diagnostics requirements for light-duty vehicles
- California Air Resources Board — On-Board Diagnostic (OBD) Program
General information written by CarWhere Research, not a diagnosis of your vehicle. Manufacturers can add their own sub-definitions and test conditions; confirm with the factory service information for your year, make and model. Before paying for a repair, check NHTSA for open recalls and your warranty booklet: federal law requires an emissions warranty on major emission-control parts such as the catalytic converter and engine computer.