P420 sounds like that code, 'Catalyst system below threshold.' to me. That's most likely one of your high-flow cats going bad. I've had this code come up every now and again, whenever someone brings me a car that has the cheap splice-in MIL eliminators, that are twisted together and covered in electrical tape. What that code means, is the signal between the front O2 sensors and the rear O2's, are too close to the same. The cats change the gases that come out of your engine, and the rear O2s are there to make sure the cats are doing their job. The code could be because of an O2 that is going bad, but they usually throw their own codes. I would look into clearing the code, and running a full tank of mid-grade gas. Sometimes that helps. If the cat is totally dead, you could temporarily get a MIL eliminator for that sensor to get the car back into closed loop. Good luck!