What Does Code P0420 Mean on a 2020 Subaru Legacy?
A P0420 code on your 2020 Subaru Legacy means the ECU has detected that the Bank 1 catalytic converter is underperforming. The 2020 Legacy was the first year of the seventh generation, featuring either the standard 2.5L FB25 boxer-4 producing 182 horsepower or the optional turbocharged 2.4L FA24DIT boxer-4 delivering 260 horsepower in the XT trim. Both use the Lineartronic CVT and standard Symmetrical AWD.
At this age with potentially 50,000-80,000+ miles, the P0420 could indicate either genuine converter wear or sensor-related issues. A thorough diagnosis is essential before committing to an expensive repair.
Common Symptoms
- Steady check engine light
- Slight decrease in fuel economy from the rated 27 city/35 highway MPG
- Rotten egg or sulfur smell from the tailpipe
- Emissions test failure
- Usually no major performance concerns
Most Likely Causes on the 2020 Legacy
1. Failing Catalytic Converter
The FB25 engine in the standard Legacy can experience catalytic converter degradation, particularly if it has been consuming oil. The converter on the Legacy is positioned underneath the vehicle where it is exposed to road conditions and temperature cycling.
2. Faulty Downstream O2 Sensor
The rear oxygen sensor on Bank 1 can become sluggish after years of use. Corrosion at the sensor connector is common, particularly in regions with road salt.
3. Exhaust Leak
Boxer engine exhaust header gaskets are a traditional weak point on Subarus. A small leak at the header or converter flange allows air into the exhaust stream, throwing off catalyst monitoring.
4. Oil Consumption Affecting Catalyst
The FB25 engine in some Legacy units can consume oil between changes. This oil residue passing through the converter gradually poisons the catalyst material.
How to Diagnose the P0420 Code
- Scan for all diagnostic trouble codes — check for companion codes
- Check for Subaru TSBs for the 2020 Legacy platform
- Monitor live O2 sensor data on Bank 1 using a scan tool
- Inspect the exhaust system for leaks from the manifold to the tailpipe
- Measure converter inlet and outlet temperatures to verify efficiency
Repair Options and Costs
Professional Repair
- Catalytic converter replacement: $800–$1,600 parts and labor
- Downstream O2 sensor: $140–$280
- Exhaust header gasket repair: $200–$500
- Spark plug replacement: $120–$250
DIY Considerations
O2 sensor replacement is manageable for home mechanics. The Legacy has decent ground clearance for a sedan, making access easier than some cars. Converter replacement is moderate difficulty. Check if the vehicle is still under the 8-year emissions warranty.
Cost Breakdown Summary
- OEM catalytic converter: $400–$1,050
- Aftermarket converter: $170–$450
- O2 sensor (part): $50–$130
- Labor for converter: $250–$500
- Diagnostic fee: $80–$150
Prevention Tips
- Monitor oil consumption on the FB25 engine — check the dipstick regularly
- Use 0W-20 synthetic oil changed every 6,000 miles
- Replace spark plugs at the recommended interval
- Avoid frequent short trips that do not allow the exhaust to reach operating temperature
- Use Top Tier gasoline for clean combustion