I have a 99 sport with the 3.9 auto and this code comes up from time to time but not consitientley, only once in a blue moon and then only for one driving cycle at a time. it dosn't seem to be coming up with any running problems or gas milage drops, just comes and goes randomly. the irriatation factor is the biggest problem I'm having with it. what could be the cause and how do you fix it?

 

I think its your cat. I would get a new one. or you can try replacing your O2 sensor after the cat.

P0420 (M) 1/1 Catalytic Converter Efficiency Catalyst 1/1 efficiency below required level.