P0408 isnt a cat code, its an EGR code...P0430 is the code that fits your question? What code do you actually have?
If it is P0430 Then it could have been caused by the misfiring, check the cat for damage to the inside, or honeycomb bed. Rap on it and see if it rattles, if it does the bed has broken up. Heres two pictures of the cat and O2 sensors before and after low effeciency is when the rear sensor switches fast, like the front one, an efficent system switches slow and is almost flat on the rear sensor.
If you have P0408 let me know, there are a few Tech bullitens on this...
Sunday, November 15th, 2009 AT 6:32 AM