P0135 Subaru Forester
O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1) · Powertrain Code · Low · Repair: Easy
What is P0135?
The heater circuit in the upstream O2 sensor on bank 1 has failed, delaying sensor warm-up.
When the Subaru Forester triggers code P0135 (o2 sensor heater circuit malfunction (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 easy, meaning basic tools, no lift needed.
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
- Burned-out O2 sensor heater element (common failure on Subaru)
- Blown O2 heater fuse
- Corroded O2 sensor connector from road spray under the boxer engine
- O2 sensor heater relay failure
Symptoms
- Check Engine Light on
- Extended warmup to closed loop
- Slightly rich running when cold
- Failed emissions test
Notes for Subaru Forester
O2 sensor heater circuit failures are common on Subaru due to the exposed location under the boxer engine; use quality sensors.