Code dictionary Generic OBD-II
P0420Catalyst System Efficiency Below Threshold Bank 1
This is a definition, not a repair instruction. It does not identify a failed part or establish that a vehicle is safe to drive.
Reference scope
Generic OBD-II definition.
A manufacturer label does not establish the model, year, engine or module to which a diagnosis applies. Read the safety and diagnostic limits.
Separately researched
Vehicle-specific diagnosis
These guides address the exact vehicles and conditions in their sources. Select the guide that matches your vehicle.
Chevrolet / GMC
Silverado and Sierra P0420/P0430: check the airbox before exhaust parts
Look for an intake sealing failure before authorizing catalyst or oxygen-sensor work.
Volkswagen
2018 VW Atlas P0420/P0430: why the catalyst and software repair go together
Identify the applicable Atlas bulletin and distinguish catalyst faults from the separate software-only code group.
Definition source
- DTC Database: pinned community code definitions
Source checked · MIT