P0135 on a Subaru: Causes, Fixes & Repair Cost
🟠 Moderate severity · drivable
Safe to drive, but the sensor warms up slowly and fuel economy and emissions may suffer until the heater circuit is fixed.
What P0135 means on a Subaru
On a Subaru, P0135 means the heater inside the bank 1 upstream oxygen sensor isn't working. The sensor is mounted in the exhaust before the catalytic converter and uses its heater to reach working temperature quickly. With a dead heater, the sensor reads slowly and the engine may run rich while warming up.
Is P0135 common on Subarus?
P0135 is common on higher-mileage Subarus like the Outback, Forester, and Impreza, where oxygen sensors are a known wear item. The heater element failing inside the sensor is the usual cause. On boxer engines the sensor can sit in a tight spot, so access adds a bit of labor time at a shop.
Commonly reported on: Outback, Forester, Impreza, Crosstrek.
Symptoms
- Check engine light is on
- Slower sensor warm-up after cold start
- Slightly worse fuel economy
- Possible failed emissions test
Common causes
- most common: Failed O2 sensor heater element
- common: Blown heater circuit fuse or relay
- common: Damaged or corroded wiring or connector at the sensor
- less common: Defective aftermarket sensor
How to fix it
| Fix | DIY? | Typical cost |
|---|---|---|
| Replace the upstream oxygen sensor | Yes | $90–230 |
| Replace the blown fuse or relay | Yes | $10–40 |
| Repair the wiring or connector | Mechanic | $80–220 |
Frequently asked questions
Other common Subaru codes
P0135 on other brands
Looking for the full (non-brand-specific) guide? See the P0135 code page →
Browse all Subaru codes: Subaru hub →
Updated 2026-08. This page is informational and not a substitute for professional diagnosis.