What Does Code P0420 Mean on a 2022 Subaru Impreza?
A P0420 code on your 2022 Subaru Impreza signals that the Bank 1 catalytic converter is not meeting efficiency standards. The 2022 Impreza is powered by a 2.0L FB20 horizontally-opposed boxer-4 engine producing 152 horsepower and 145 lb-ft of torque, mated to the Lineartronic CVT with standard Symmetrical AWD.
The ECU monitors catalytic converter performance by comparing voltage signals from the upstream and downstream oxygen sensors. A healthy converter results in the downstream sensor showing a stable, relatively flat signal. When it starts mimicking the upstream sensor's rapid fluctuations, the ECU triggers P0420.
Common Symptoms
- Check engine light illuminated (steady)
- Reduced fuel economy from the expected 28 city/36 highway MPG
- Sulfur smell from the tailpipe
- Emissions test failure
- Generally no noticeable performance loss
Most Likely Causes on the 2022 Impreza
1. Failing Catalytic Converter
Even on a relatively young vehicle, the catalytic converter can begin showing wear, especially if the FB20 engine has been consuming oil. Oil contamination is one of the faster routes to converter degradation on Subaru boxers.
2. Faulty Downstream O2 Sensor
A sluggish or defective rear oxygen sensor can trigger P0420 even when the converter is fine. Sensor contamination from oil or moisture is a known concern on the boxer engine platform.
3. Exhaust Leak
Subaru's boxer exhaust headers connect to the converter through gaskets that can fail. Even a small leak allows unmetered oxygen into the exhaust, fooling the ECU into thinking the converter is underperforming.
4. Software Calibration
Subaru has issued ECU updates for various models that adjust the P0420 monitoring thresholds. A software update may resolve the code without any physical repair.
How to Diagnose the P0420 Code
- Scan for all stored and pending codes
- Check for Subaru TSBs and ECU updates for the 2022 Impreza
- Compare upstream and downstream O2 sensor live data waveforms
- Inspect the exhaust for leaks from the headers down to the converter outlet
- Check oil consumption records — if adding oil between changes, note the consumption rate
Repair Options and Costs
Professional Repair
- Catalytic converter replacement: $700–$1,400 parts and labor
- O2 sensor replacement: $130–$260
- Exhaust leak repair: $150–$400
- ECU software update: $100–$180 at the dealer
DIY Considerations
O2 sensor replacement is manageable with basic tools. The Impreza is lower to the ground than the Forester or Outback, so good jack stands are important. Check emissions warranty before paying for any repairs.
Cost Breakdown Summary
- OEM catalytic converter: $350–$900
- Aftermarket converter: $150–$400
- O2 sensor (part): $45–$120
- Labor: $150–$450
- Diagnostic fee: $80–$130
Prevention Tips
- Monitor oil consumption on the FB20 engine closely
- Use 0W-20 synthetic oil and change every 6,000 miles
- Replace spark plugs at the recommended interval to prevent misfires
- Allow the engine to reach full operating temperature on every drive
- Use quality Top Tier gasoline