2018 Honda P0420: What It Means & Is It Safe to Drive?
P0420 on the 2018 Honda: Catalyst System Efficiency Below Threshold (Bank 1). Here's what it means, what usually causes it, what repairs typically cost, and whether it's safe 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 on the 2018 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 2018 Honda, repairs for P0420 typically fall between $400 and $2400, depending on the root cause and labor rates in your area.
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