What Does Code P0420 Mean on a 2018 Subaru Outback?
A P0420 code on your 2018 Subaru Outback indicates that the Bank 1 catalytic converter is not cleaning exhaust gases efficiently. The 2018 Outback features either the 2.5L FB25 boxer-4 engine producing 175 horsepower and 174 lb-ft of torque, or the optional 3.6L EZ36 boxer-6 with 256 horsepower. Both use Subaru's Symmetrical AWD system with a Lineartronic CVT transmission.
At this age, with potentially 60,000-100,000+ miles, the P0420 code is more likely to indicate genuine catalytic converter wear rather than a sensor or software issue. However, all possibilities should be explored before committing to an expensive converter replacement.
Common Symptoms
- Check engine light on (steady)
- Reduced fuel economy from the expected 25 city/32 highway MPG
- Rotten egg or sulfur smell from the tailpipe
- Emissions test failure
- Possible sluggishness if the converter is becoming clogged
Most Likely Causes on the 2018 Outback
1. Worn Catalytic Converter
The 2018 Outback's catalytic converter has been through years of heat cycling and is at the age where converter failure becomes common. The FB25 boxer engine is known for oil consumption issues that can accelerate converter deterioration as oil coats the catalyst substrate.
2. Faulty Downstream O2 Sensor
The post-converter O2 sensor can become sluggish with age and mileage. On the boxer engine, this sensor sits low and is exposed to road conditions that can corrode the connector and wiring.
3. Exhaust Leak
The 2018 Outback's exhaust system may have developed corrosion or gasket failure after years of use. Subaru boxer engines are known for exhaust manifold and header gasket issues that can cause leaks affecting O2 sensor readings.
4. Oil Consumption Damaging the Catalyst
The FB25 engine in the 2018 Outback has a documented history of excessive oil consumption. Oil burning through the combustion chamber contaminates the catalytic converter, gradually reducing its efficiency.
How to Diagnose the P0420 Code
- Scan for all stored and pending codes — check for misfire codes, O2 sensor codes, and oil-related codes
- Check oil consumption history — if you are adding oil between changes, this is a clue
- Monitor upstream and downstream O2 sensor live data on Bank 1
- Perform a thorough exhaust system inspection for leaks, corrosion, and damaged gaskets
- Test catalytic converter efficiency using inlet/outlet temperature differential
Repair Options and Costs
Professional Repair
- Catalytic converter replacement: $800–$1,600 parts and labor
- Downstream O2 sensor replacement: $140–$280
- Exhaust header gasket repair: $200–$500
- Oil consumption repair (piston rings): $2,500–$4,000 if this is the root cause
DIY Considerations
O2 sensor replacement is a reasonable DIY job. Catalytic converter replacement on the boxer engine requires working underneath the vehicle and dealing with potentially corroded bolts, making it moderately difficult. If oil consumption is the root cause, that is definitely a professional repair.
Cost Breakdown Summary
- OEM catalytic converter: $400–$1,000
- Aftermarket converter: $150–$450
- O2 sensor (part): $50–$130
- Labor for converter install: $200–$500
- Diagnostic fee: $80–$150
Prevention Tips
- Monitor oil consumption closely — check the dipstick every 1,000 miles and document consumption
- Use 0W-20 synthetic oil and change it every 6,000 miles
- Replace spark plugs at the recommended 60,000-mile interval
- Address any engine misfires immediately to protect the catalytic converter
- If oil consumption exceeds 1 quart per 1,200 miles, consult a Subaru specialist about a potential warranty extension or repair