What Does Code P0420 Mean on a 2022 Subaru Outback?
If your 2022 Subaru Outback is displaying a P0420 code, the ECU has determined that the Bank 1 catalytic converter is not efficiently converting harmful exhaust emissions. The 2022 Outback features either the standard 2.5L FA25 boxer-4 engine producing 182 horsepower or the turbocharged 2.4L FA24 boxer-4 making 260 horsepower in the XT trim. Both engines use Subaru's signature horizontally-opposed layout with Symmetrical AWD.
Subaru boxers are known for their unique exhaust note and configuration. The flat engine layout means the exhaust headers route differently than conventional engines, which affects how the catalytic converter sits in the exhaust system and how it wears over time.
Common Symptoms
- Steady check engine light (not flashing)
- Mild decrease in fuel economy
- Sulfur or rotten egg smell from exhaust
- Emissions test failure
- Typically no significant performance changes
Most Likely Causes on the 2022 Outback
1. Failing Catalytic Converter
The 2022 Outback's catalytic converter can begin to degrade especially if the vehicle has been subjected to many short trips or cold-weather idling. Subaru's boxer engines run at slightly different exhaust temperatures than inline configurations, which can affect converter longevity.
2. Faulty Downstream O2 Sensor
The post-converter oxygen sensor on the boxer engine sits in a lower position, making it more susceptible to road splash and corrosion. A sluggish or failed sensor can trigger P0420 without an actual converter problem.
3. Exhaust Header Gasket Leak
Subaru boxers are known for exhaust header gasket issues. A leak at the header-to-converter junction on Bank 1 can allow unmetered air into the exhaust stream, confusing the O2 sensors.
4. Oil Consumption Affecting Catalyst
Some Subaru boxer engines can consume oil, and oil vapor passing through the combustion chamber can coat and damage the catalytic converter substrate over time.
How to Diagnose the P0420 Code
- Scan for all codes including pending codes — look for O2 sensor or misfire codes
- Check for Subaru TSBs for the 2022 Outback regarding P0420 or ECU calibration
- Monitor live O2 sensor data on Bank 1 — compare upstream and downstream waveforms
- Inspect the boxer exhaust system for leaks, especially at header gaskets and converter flanges
- Check oil consumption records — excessive oil burning can damage the converter
Repair Options and Costs
Professional Repair
- Catalytic converter replacement: $900–$1,800 parts and labor
- Downstream O2 sensor replacement: $150–$300
- Exhaust header gasket repair: $200–$500 (labor-intensive on boxer engines)
- ECU software update: $100–$200 at dealer
DIY Considerations
O2 sensor replacement is doable on the Outback with proper jack stands and a sensor socket. Exhaust header gasket work on a boxer engine is significantly more involved due to the engine layout and is best left to a Subaru specialist. Check warranty coverage first — the 2022 may still be within emissions warranty.
Cost Breakdown Summary
- OEM catalytic converter: $500–$1,200
- Aftermarket converter: $200–$500
- O2 sensor (part): $60–$150
- Header gasket set: $30–$80 (parts), $200–$400 (labor)
- Diagnostic fee: $100–$150
Prevention Tips
- Use 0W-20 synthetic oil and change it every 6,000 miles per Subaru's recommendation
- Monitor oil consumption between changes — add oil if the level drops below the full mark
- Replace spark plugs at the recommended interval to prevent misfires
- Allow the engine to warm up properly before driving hard, especially in cold weather
- Use Top Tier gasoline to keep injectors and combustion chambers clean