What Does Code P0420 Mean on a 2020 Subaru Ascent?
A P0420 code on your 2020 Subaru Ascent indicates that the Bank 1 catalytic converter is not efficiently cleaning exhaust gases. The Ascent is powered exclusively by a turbocharged 2.4L FA24DIT boxer-4 engine producing 260 horsepower and 277 lb-ft of torque, paired with a Lineartronic CVT and standard Symmetrical AWD. This is Subaru's largest vehicle, designed for family hauling and light towing.
The turbo engine runs at higher exhaust temperatures than naturally aspirated boxers, which affects how the catalytic converter wears over time. At around 50,000-80,000 miles, converter issues become more common.
Common Symptoms
- Check engine light illuminated
- Reduced fuel economy from the expected 21 city/27 highway MPG
- Sulfur or rotten egg smell from the exhaust
- Emissions test failure
- Usually no significant performance changes
Most Likely Causes on the 2020 Ascent
1. Failing Catalytic Converter
The turbocharged 2.4L engine produces higher exhaust gas temperatures than naturally aspirated Subaru boxers. This additional heat stress, combined with frequent family-hauling loads, can accelerate converter degradation. If the Ascent is used for towing (rated up to 5,000 lbs), converter wear increases further.
2. Faulty Downstream O2 Sensor
The post-converter oxygen sensor on the turbo boxer can fail prematurely due to heat exposure and soot contamination. The turbo engine produces more exhaust byproducts that can coat sensor elements.
3. Exhaust Leak
The turbo engine's exhaust system includes the turbocharger, downpipe, and catalytic converter in a more complex routing than a naturally aspirated setup. Gasket failures at any connection point can cause P0420.
4. Turbo-Related Issues Affecting Catalyst
A failing wastegate or boost leak can alter the air-fuel ratio, sending excess fuel through the converter and potentially damaging it. Check for boost-related codes alongside P0420.
How to Diagnose the P0420 Code
- Scan for all codes — pay special attention to boost, misfire, and O2 sensor codes
- Check for Subaru TSBs specific to the 2020 Ascent turbo engine
- Monitor live data from upstream and downstream O2 sensors
- Inspect the turbo downpipe, wastegate, and all exhaust connections for leaks
- Test catalytic converter efficiency with temperature measurements
Repair Options and Costs
Professional Repair
- Catalytic converter replacement: $1,000–$1,900 parts and labor (turbo models cost more)
- Downstream O2 sensor: $160–$310
- Exhaust leak repair: $150–$450
- Turbo system diagnosis and repair: $200–$800 depending on issue
DIY Considerations
The Ascent's turbo exhaust system is complex, and DIY work is not recommended for converter or turbo-related repairs. O2 sensor replacement is feasible for experienced mechanics. The vehicle may still be under emissions warranty — check first.
Cost Breakdown Summary
- OEM catalytic converter: $600–$1,300
- Aftermarket converter: $250–$600
- O2 sensor (part): $60–$170
- Labor: $300–$600
- Diagnostic fee: $100–$150
Prevention Tips
- Use 5W-30 full synthetic oil as specified for the turbo FA24 engine and change every 6,000 miles
- Allow the turbo to cool down after hard driving by idling for 30-60 seconds before shutdown
- If towing with the Ascent, allow extra cool-down time to reduce exhaust system heat stress
- Replace spark plugs at the recommended interval — misfires are especially damaging to turbo converters
- Use premium 91+ octane fuel as recommended by Subaru for the turbo engine