What does code P0420 mean on a Subaru and is it safe to drive?
P0420: Catalyst System Efficiency Below Threshold (Bank 1). Here's what it means on your Subaru, what usually causes it, what repairs typically cost, and how safe it is to keep driving.
What this code means
The downstream oxygen sensor detects that the catalytic converter is not storing or converting exhaust properly. This is one of the most common check engine codes.
Common causes for your Subaru
- Failed or aging catalytic converter
- Faulty downstream oxygen sensor
- Exhaust leak before or after the catalyst
- Engine misfire or rich/lean condition damaging the catalyst
Symptoms
- Check Engine Light on
- Often no drivability issues
- Possible failed emissions test
- Rotten egg smell in some cases
Typical fixes
- Replace catalytic converter
- Replace oxygen sensor(s)
- Repair exhaust leaks
- Fix underlying fuel/ignition issues first
Repair cost range
For a Subaru, repairs for P0420 often fall between $400 and $2400, depending on the cause and whether you use OEM or aftermarket parts. Labor rates vary by location.
Known issues on Subaru
Subaru P0420 on the EJ25 2.5L engine is almost always connected to the infamous head gasket failure. Coolant entering the combustion chamber poisons the catalyst rapidly—if you see P0420 alongside white exhaust smoke or coolant loss, do a head gasket diagnosis before spending money on a catalyst. The cat itself is often collateral damage. On turbocharged WRX/STI EJ257 engines, oil consumption via turbo seals also destroys cats; replace the turbo seal before the converter.
Most commonly affected models