Hey Guys, I am new to the board and I have a question. I have a 2003 Cadillac cts with a check engine light. I took it to Auto Zone and had it scanned and it has two codes. One is a po420 catalyst efficiency below threshold bank 1 and a po430 catalyst efficiency below threshold bank 2. What could be the causes? It did the same thing about 2 weeks ago and the light reset itself. It came back on a few days later with the same codes. btw it has 94,000 miles. Thanks pw

Fairly complex diagnosis, lots of of info on the forums though, do a site search for the p0420.

I did!!! 3 posts and one is mine. Not much info.

Cat efficiency is very common, there are a multitude of articles, lengthy, addressing the issue try googling, I could run you by my experiences, if you still cannot find them.

Hey Max, I googled and got either p0420 or p0430 but not both together. What could cause both to come on at the same time? Thanks PW

Go here.....

http://www.obd-codes.com/

Look up both codes then compare causes for commonality.

The best way to check cat efficiency beyond the pcm checking the pre and post sensor inputs, is to get the car good and hot by driving for 1/2 hour, then check the inlet temp at the cat and compare it to the outlet temp, using a non contact thermo, the difference should be at least 100 degrees more at the outlet. Usually both codes are caused by the same issues.