What Does Code P0420 Mean on a 2024 Subaru Forester?
A P0420 code on your 2024 Subaru Forester means the ECU has detected that the Bank 1 catalytic converter is underperforming. The 2024 Forester is equipped with a 2.5L FA25 horizontally-opposed boxer-4 engine producing 182 horsepower and 178 lb-ft of torque, paired with a Lineartronic CVT and standard Symmetrical AWD. On a brand-new Forester, this code is unusual and likely points to a warranty-covered issue.
The ECU compares signals from the upstream and downstream oxygen sensors. When the converter is healthy, the downstream sensor produces a stable, flat signal. If it starts mimicking the rapid switching of the upstream sensor, the ECU flags P0420.
Common Symptoms
- Check engine light illuminated
- Minimal noticeable performance changes
- Slight decrease in fuel economy from the rated 26 city/32 highway MPG
- Emissions test failure
- Possible faint sulfur odor
Most Likely Causes on the 2024 Forester
1. ECU Software Calibration Issue
On a new 2024 Forester, the most probable cause is a software calibration issue. Subaru periodically releases ECU updates that refine catalyst monitoring parameters for the FA25 engine.
2. Faulty Downstream O2 Sensor
A factory-defective oxygen sensor downstream of the catalytic converter can send erratic readings, triggering a false P0420. The boxer engine positions this sensor lower, where it can be affected by moisture.
3. Exhaust System Assembly Issue
A loose connection or improperly seated gasket from the factory can create a small exhaust leak that affects O2 sensor readings and triggers the code.
4. Catalytic Converter Manufacturing Defect
Though rare on a new vehicle, a defective converter with insufficient catalyst coating is possible and would be warranty-replaced.
How to Diagnose the P0420 Code
- Bring the vehicle to an authorized Subaru dealer for warranty diagnosis
- The dealer will check for applicable TSBs and ECU software updates
- Live data analysis of both O2 sensors on Bank 1 will identify sensor issues
- Exhaust system inspection for assembly defects or leaks
- Catalytic converter efficiency testing if other causes are eliminated
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 Cost Reference
- Catalytic converter replacement: $800–$1,600
- O2 sensor replacement: $150–$280
- ECU update at dealer: $100–$200
- Exhaust leak repair: $100–$300
DIY Considerations
Do not perform DIY repairs on a 2024 Forester. The vehicle is under full warranty and all emissions-related repairs should be handled by a Subaru dealer at no cost.
Cost Breakdown Summary
- Under warranty: $0 for all P0420-related repairs
- OEM converter (future reference): $450–$1,100
- O2 sensor part: $60–$150
Prevention Tips
- Follow Subaru's maintenance schedule with 0W-20 synthetic oil every 6,000 miles
- Keep ECU software current through dealer service visits
- Use Top Tier gasoline as recommended by Subaru
- Address check engine lights promptly — do not ignore them
- Keep all service records for warranty documentation