2001 Cadillac DeVille, engine light & p0420, has rough idle 400 shuts off at stop, cause?

Question

2001 Cadillac DeVille DHS, engine light & p0420, has rough idle 400 shuts off at a stop, cause? No other symptom

Answer 1

P0420 Catalytic below threshold

In order for the catalytic convertor to work the it needs to go from lean to rich , with a rough idle or even a vacuum leak or lean misfire you will get P0420, this may not mean you need a catalytic, you need to resolve the rough idle, This can be from a idle speed actuator to a partially stuck open EGR valve.

Answer 2

P0420 CADILLAC - Catalyst System Low Efficiency Bank 1

Read more: https://www.autocodes.com/p0420_cadillac.html