I have a 2001 GT Eclipse and my check engine light was checked at the dealer and the codes PO421 & PO141 came up.
Prior to this I had a PO136 code as well but I had my oxygen sensor changed at the shop I usually go to and that took care of that one. But the 141 code stayed and then the 421 code showed up after that. So, after clearing the codes and having the light stay on I figured I'd take it to the dealer and just have them find out what was wrong.

They told me it's the catalytic converter plus the bank 1 sensor 2 O2 sensor again as well. They mentioned that the cat could be overheating and destroying the new sensor, great. They also said a new cat converter would cost $1268, just for the part. The whole job is apparently going to cost $1685 plus tax. Is this right or should it be cheaper? I know dealers usually mark parts and labor up a bit. I'm hoping that's the case here as this is just to get my car inspected.
They mentioned that they've had problems installing aftermarket catalytic converters and that you can't use a used one in NY State because they will fail inspection.
My car has been running fine except for some rough idling from time to time due to the O2 sensor perhaps? The gas mileage has actually increased throughout this whole ordeal. Any advice or thoughts would be appreciated! i'm hoping to pay even a little bit less than what the dealer quoted me.

- Dave

 

I had the same problem as yours, my car is mitsu lancer ES 2003. error is P0421

did your fix your problem?