hehe, my WRX throws P0420 sometimes. Catalytic convertor inefficiency. Mine is because I removed one of the 3 cats in the car. If the car is unmodified, I doubt the cat would be burned up. Might just be a bad rear O2, or the car running very rich.

Intake valve sensor? Sounds like a trip to Nissan for a new one. Does that car have some sort of variable cam thing?

I'd get the valve sensor diagnosed first. It may be that its causing the P0420.