OK,
So I've been riding around with this code for about a year! At first I cleared everything with my scanner, it came back. About 3-4 months ago I finally broke down and put in a new Magnaflow High Flow Cat. I ordered the wrong one apparently. Everything I read on this board and other places said for an 03 you want the cat with the O2 bung in it. Well, I should have gotten under and looked first. The O2 sensor is after the cat, further down the pipe on my car. The guy at the shop said he could just seal up the hole and weld it. He seemed to know what he was doing (I should have and would have ordered the magnaflow from him if I had found him first).

After about 2 weeks of driving with the new cat in, the CEL came on again! I figured, OK- the rear O2 is bad because of crap getting sent down the tube. So, I finally got around to looking at it this past weekend. I put in 2 new Denso O2 sensors and cleared the code. Everything was fine until yesterday. Now the light came back on. Checked the code- SAME code. What else could this be? Do you think I have a bad cat even though it'* new? I'm stumped.

Thanks in advance. Everyone here is a life saver!

It would help to have the code description from Techinfo listed...

Off to go find out what the code means.

Edit: I'm back

DTC P0420 Catalyst System Low Efficiency

This code indicates that the rear O2 reading is showing the catalytic converter to not be functioning properly.

Have you checked the wiring to both sensors for rubs or nicks in the wire coating that might allow it to ground etc? Any dents on the underside of the car in that area that might indicate a potential problem area?

Bill,
Thanks. I should have been more clear. I've been looking at the code for so long I forgot that not everyone will "just know" what it is. Stupid of me. Thanks for putting up the info.

Now that you say that- I did not get the heat sheild up the way it should be. I wonder if that'* making a difference. I don't know if it'* really hitting anything, but maybe the heat is getting to the wires and doing something? I don't know. I will try to get what I need to put the heat sheild back up this weekend. I just ran out of time.

Thanks again. Any other thoughts are appreciated.

I'd look more to the shield cutting into the wiring.