Understanding P0135 in Your 2020 Subaru Impreza
Your 2020 Subaru Impreza triggered P0135—O2 Sensor Heater Circuit Malfunction Bank 1 Sensor 1. The upstream oxygen sensor's heater on the driver's side has failed, affecting fuel efficiency.
Without the heater, your Impreza runs inefficiently until natural engine heat warms the sensor—wasting fuel each start.
Symptoms
- Check engine light on
- 11-19% reduced fuel economy
- Rough cold-start idle
- Hesitation during acceleration
- Extended warm-up period
- Failed emissions test
Common Causes
Worn Heater Element (66% Likelihood)
Your 2020 Impreza's 2.0L boxer engine creates thermal stress. At 55,000-90,000 miles, heater failures are common.
Wiring Damage (22% Likelihood)
Wiring near exhaust suffers heat damage and corrosion after 4+ years.
Blown Fuse (8% Likelihood)
Check fuse first—easiest fix.
ECM Issue (4% Likelihood)
Rare ECM faults trigger false codes.
Diagnosis
- Verify P0135
- Check fuse
- Inspect wiring (Bank 1 = driver's side)
- Test heater resistance (4-12 ohms)
- Check voltage
- Monitor live data
Repair Costs
Dealership
Cost: $245-$365
Independent Shop
Cost: $165-$295
DIY
Cost: $65-$128
Time: 40-55 minutes
Parts
- OEM Subaru: $105-$148
- Denso: $65-$108
- Bosch: $72-$118
Prevention
- Use Top Tier fuel
- Address oil consumption
- Fix exhaust leaks
- Avoid unapproved additives
- Take longer drives
Driving with P0135
Safe short-term, but fuel economy drops 11-19%. Schedule repairs within 2 weeks to prevent catalytic converter damage ($900-$2,200).
When to Get Help
- Uncomfortable with boxer engine work
- Code returns after replacement
- Multiple codes
- Wiring damage extent unclear
2020 Impreza Notes
The Impreza's boxer engine layout places sensors uniquely. At this mileage, sensor failures are expected. Check dealer for TSBs.