What Does Code P0420 Mean on a 2024 Subaru Impreza?
A P0420 code on your 2024 Subaru Impreza indicates that the ECU has detected reduced catalytic converter efficiency on Bank 1. The redesigned 2024 Impreza now features a 2.5L FA25 boxer-4 engine producing 182 horsepower and 178 lb-ft of torque — a significant upgrade from the previous 2.0L. It pairs with the Lineartronic CVT and standard Symmetrical AWD on the new Subaru Global Platform.
On a brand-new 2024 model, P0420 is unusual and almost always indicates a warranty-covered defect rather than normal wear.
Common Symptoms
- Check engine light on
- Minimal performance changes
- Slight fuel economy decrease from the rated 28 city/36 highway MPG
- Emissions test failure
- Possible faint exhaust odor
Most Likely Causes on the 2024 Impreza
1. ECU Software Calibration Issue
The 2024 Impreza is a redesigned model with the new FA25 engine. First-year calibration adjustments are common, and a software update may resolve the P0420 code without any physical repair needed.
2. Faulty Downstream O2 Sensor
A factory-defective oxygen sensor can send incorrect readings to the ECU, triggering a false P0420. This is a straightforward warranty replacement.
3. Exhaust Assembly Defect
Assembly line issues such as an improperly torqued exhaust connection or a misaligned gasket can create a small leak that throws off O2 sensor readings.
4. Catalytic Converter Manufacturing Defect
Rare on a new vehicle, but a converter with insufficient catalyst coating from the factory is possible and would be fully warranty-covered.
How to Diagnose the P0420 Code
- Take the vehicle directly to a Subaru dealer for warranty service
- The dealer will check for TSBs specific to the redesigned 2024 Impreza
- ECU software version will be verified and updated if needed
- O2 sensor live data and exhaust system inspection will be performed
- Converter efficiency testing if other causes are ruled out
Repair Options and Costs
Warranty Repair (Recommended)
- ECU software update: No cost under warranty
- O2 sensor replacement: No cost under warranty
- Exhaust repair: No cost under warranty
- Catalytic converter replacement: No cost under warranty
Out-of-Warranty Reference Costs
- Catalytic converter: $750–$1,400
- O2 sensor: $140–$270
- Exhaust leak repair: $100–$300
DIY Considerations
Do not attempt DIY repairs on a 2024 Impreza. The vehicle is under full warranty and all emissions-related work should be handled by a Subaru dealer.
Cost Breakdown Summary
- Under warranty: $0 for all P0420-related repairs
- OEM converter (future reference): $400–$950
- O2 sensor part (future reference): $55–$140
Prevention Tips
- Follow Subaru's maintenance schedule for the new FA25 engine from day one
- Use 0W-20 full synthetic oil as specified
- Keep ECU software updated by visiting the dealer for service
- Address any check engine lights promptly
- Use Top Tier gasoline as recommended by Subaru