P0131 Subaru Forester
O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1) · Powertrain Code · Medium · Repair: Moderate
What is P0131?
Low voltage from the upstream O2 sensor on bank 1, indicating a possible lean condition or sensor failure.
When the Subaru Forester triggers code P0131 (o2 sensor circuit low voltage (bank 1, sensor 1)), it typically points to an issue in the powertrain system. Below you'll find the most common causes, symptoms, and repair options specific to the Subaru Forester.
This repair is rated moderate, meaning some experience and specialty tools required.
Diagnostic data on this page is compiled from publicly available repair resources and technical service bulletins. Always have a qualified mechanic confirm the diagnosis before authorizing repairs on your Subaru Forester.
Common Causes on Subaru Forester
- Failed front O2 sensor (Bank 1 Sensor 1) on boxer engine
- Exhaust leak at header-to-head gasket (common on boxer layout)
- O2 sensor wiring damage from heat or road debris under the boxer engine
- Sensor contamination from oil consumption on FB25 engine
Symptoms
- Check Engine Light on
- Poor fuel economy
- Rough idle
- Hesitation on acceleration
- Failed emissions test
Notes for Subaru Forester
Boxer engine O2 sensors can be hard to reach from above; access from underneath on a lift is recommended.