What Does Code P0420 Mean on a 2022 Subaru Forester?
A P0420 code on your 2022 Subaru Forester indicates the ECU has detected reduced efficiency from the Bank 1 catalytic converter. The 2022 Forester is powered by a 2.5L FB25D boxer-4 engine producing 182 horsepower and 176 lb-ft of torque with direct injection. This engine pairs with the Lineartronic CVT and standard Symmetrical AWD.
The ECU monitors catalyst performance by comparing the oxygen sensor readings before and after the converter. When the downstream sensor's waveform begins matching the upstream sensor, it signals that the converter is no longer effectively processing exhaust gases.
Common Symptoms
- Steady check engine light
- Reduced fuel economy from the expected 26 city/33 highway MPG
- Sulfur or rotten egg smell from exhaust
- Failed emissions test
- Usually no major drivability concerns
Most Likely Causes on the 2022 Forester
1. Failing Catalytic Converter
The 2022 Forester's catalytic converter can begin to degrade, especially with frequent short-trip driving that does not allow the exhaust system to reach optimal operating temperature. The FB25D's direct injection can also contribute to carbon deposits that affect the converter.
2. Faulty Downstream O2 Sensor
The rear oxygen sensor on the boxer engine can become sluggish or contaminated with age. Its lower position on the Forester exposes it to water and road debris.
3. Exhaust Leak
Subaru's boxer exhaust header gaskets can develop leaks, allowing extra air into the exhaust stream and confusing the O2 sensor comparison that monitors catalyst efficiency.
4. Engine Misfire or Oil Consumption
Some FB25 engines can consume oil between changes. Oil passing through the combustion process can foul the catalytic converter over time. Any misfires compound this problem by sending unburned fuel into the converter.
How to Diagnose the P0420 Code
- Scan for all diagnostic codes — look for accompanying misfire or O2 sensor codes
- Check for Subaru TSBs related to P0420 on the 2022 Forester
- Monitor live O2 sensor data to compare upstream and downstream waveforms
- Inspect the exhaust system for leaks, focusing on header gaskets and converter connections
- Test catalytic converter efficiency using temperature differential measurement
Repair Options and Costs
Professional Repair
- Catalytic converter replacement: $850–$1,650 parts and labor
- Downstream O2 sensor replacement: $150–$290
- Exhaust header gasket repair: $200–$500
- Spark plug replacement: $120–$250
DIY Considerations
O2 sensor replacement is manageable for experienced DIYers. The Forester has reasonable ground clearance for accessing the sensor. Catalytic converter and header gasket work is more involved due to the boxer engine layout. Check if the vehicle is still under emissions warranty first.
Cost Breakdown Summary
- OEM catalytic converter: $450–$1,100
- Aftermarket converter: $180–$450
- O2 sensor (part): $55–$140
- Labor for converter install: $250–$550
- Diagnostic fee: $80–$150
Prevention Tips
- Use 0W-20 synthetic oil and change it every 6,000 miles
- Monitor oil consumption between changes — add oil as needed
- Replace spark plugs at the recommended 60,000-mile interval
- Take longer drives periodically to allow the exhaust system to fully heat up and burn off deposits
- Use Top Tier gasoline for cleaner combustion