I  picked up another free Subaru because of rust issues, it is a 2000 impreza with a 2.2 150 k on engine. It will be strictly an off road beater because of the rust.

 I had the rear cat cut out because it had rust holes in it and had a straight pipe welded in but now I get a p0420 code. I understand that the code has to do with catalyst but was wondering if it will do any damage to the front cat or engine if I drive it like that. I would also like to know if there is a fix like the spark plug fouler trick that would turn that code off? 

the ECU does not alter any engine operation parameters at all from that sensor's data.

 

anti-fouler extension might work. I think there is a resister mod that might work too.

Won't hurt it.

Only needs the front sensor for air/fuel ratio changes.