What Does Code P0420 Mean on a 2020 Subaru Impreza?
A P0420 code on your 2020 Subaru Impreza means the ECU has flagged the Bank 1 catalytic converter for reduced efficiency. The 2020 Impreza uses a 2.0L FB20 horizontally-opposed boxer-4 engine producing 152 horsepower and 145 lb-ft of torque, paired with a Lineartronic CVT and standard Symmetrical AWD.
The 2020 Impreza at this point may have 40,000-80,000 miles on it, placing it in the range where both sensor and converter issues become relevant. Subaru's boxer engines are generally reliable, but the exhaust system requires attention at this mileage.
Common Symptoms
- Steady check engine light
- Decreased fuel economy from the rated 28 city/36 highway MPG
- Sulfur or rotten egg smell from exhaust
- Emissions test failure
- Usually minimal drivability impact
Most Likely Causes on the 2020 Impreza
1. Failing Catalytic Converter
The Impreza's catalytic converter can begin to degrade at higher mileage. The FB20 engine's known oil consumption tendency can accelerate converter wear as oil residue coats the catalyst substrate.
2. Faulty Downstream O2 Sensor
The rear oxygen sensor on the 2.0L boxer can become sluggish with age. Road salt, moisture, and debris exposure accelerate sensor deterioration, especially in northern climates.
3. Exhaust Leak
The boxer engine's exhaust manifold gaskets and the connection between the header and catalytic converter can develop leaks. These are known weak points on Subaru boxers.
4. Engine Misfires
Worn spark plugs or failing ignition coils on the FB20 can cause misfires that send unburned fuel into the catalytic converter, overheating and damaging it.
How to Diagnose the P0420 Code
- Pull all stored and pending codes — look for misfire or O2 sensor companion codes
- Check for Subaru TSBs for the 2020 Impreza regarding P0420
- Monitor upstream and downstream O2 sensor live data on Bank 1
- Inspect the exhaust system for visible leaks and corrosion
- Test catalytic converter inlet vs. outlet temperature with an infrared thermometer
Repair Options and Costs
Professional Repair
- Catalytic converter replacement: $700–$1,400 parts and labor
- Downstream O2 sensor: $130–$260
- Exhaust header gasket repair: $180–$400
- Spark plug replacement: $100–$200
DIY Considerations
The Impreza sits lower to the ground than the Crosstrek or Forester, making exhaust access tighter. O2 sensor replacement is still feasible with a jack, stands, and a sensor socket. Converter replacement is a moderate-difficulty job best suited for experienced home mechanics.
Cost Breakdown Summary
- OEM catalytic converter: $350–$900
- Aftermarket converter: $150–$400
- O2 sensor (part): $45–$120
- Labor for converter: $200–$450
- Diagnostic fee: $80–$130
Prevention Tips
- Check oil level every 1,000 miles — the FB20 can consume oil between changes
- Use 0W-20 synthetic oil changed every 6,000 miles
- Replace spark plugs at the recommended 60,000-mile interval
- Avoid excessive short trips that do not allow the exhaust to reach full temperature
- Use Top Tier gasoline for cleaner combustion and reduced carbon buildup