What does code P0420 mean on a Honda and is it safe to drive?
P0420: Catalyst System Efficiency Below Threshold (Bank 1). Here's what it means on your Honda, 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 Honda
- 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 Honda, 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 Honda
Honda P0420 on the 2.4L K24 engine often precedes or follows oil consumption issues—burning oil poisons the catalyst. On Civic 1.5T (2016–2021), the known oil dilution problem from short-trip driving in cold climates can destroy the catalytic converter prematurely; Honda issued an extended warranty on affected models. Confirm the downstream O2 sensor (sensor 2) is functioning correctly before replacing the converter—Honda replacement cats should be OEM or direct-fit to pass CARB states.
Most commonly affected models