2002 Nissan Altima TROUBLE CODE P0420

The converter efficiency isn't alsway affected by if the bed is damaged or not, the precious metels inside it, platinum and paladium can be burned out of it, and you can't tell by looking at it, it is judged by how much switching is done by the downstream O2 sensor, and yours must be switching between rich and lean too much.

So how much does this normally cost to fix