P0420 · Catalyst efficiency below threshold, bank 1
The computer measured catalyst performance below its limit on the side of the engine that includes cylinder 1.
Part of the Catalyst system guide.
What the monitor saw
The catalyst monitor ran under its required conditions and reported low efficiency for bank 1. A single-bank engine only has bank 1. On an engine with two banks, the cylinder layout tells you which side is bank 1.
What to check first
Address active misfires or fuel-control problems. Then ask for an exhaust leak check and a review of the oxygen or air-fuel sensor data using the correct vehicle procedure. If the converter did fail, find out what may have damaged it before fitting another one.
Why the vehicle matters
Volkswagen bulletin 01-20-01, for example, gives a combined catalyst and software repair for a defined group of 3.6L Atlas vehicles. Check the VIN and engine before applying a bulletin.