P0135low severityPowertrain
O2 Sensor Heater Circuit Bank 1 Sensor 1
on 2004 Subaru Legacy 3.0L H6 · AWD
What this code means
The PCM detected a malfunction in the heater element of the upstream O2 sensor on Bank 1.
Symptoms
- Check Engine Light
- Extended warmup emissions
- Failed emissions test
Common causes
- Faulty O2 sensor heater element
- Blown fuse
- Wiring damage
- PCM heater driver failure
Diagnostic steps
- Check O2 heater fuse
- Test heater resistance (typically 2-30 ohms)
- Inspect wiring
- Check PCM heater relay output
Repair notes
Replace O2 sensor if heater resistance is open or out of spec.
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