gone12 Posted March 24, 2008 Report Share Posted March 24, 2008 my sls has a code po420,,,,catalyst system low efficiency. or is the rear oxygen sensor dead?? Link to comment Share on other sites More sharing options...
BodybyFisher Posted March 24, 2008 Report Share Posted March 24, 2008 Sounds like a CAT problem, did you change the cat? Are you having any performance issues? Running rich? Pre-1995 - DTC codes OBD1 >> 1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/ Cadillac History & Specifications Year by Year http://www.motorera.com/cadillac/index.htm Link to comment Share on other sites More sharing options...
gone12 Posted March 24, 2008 Author Report Share Posted March 24, 2008 car runs good,has the stock cat in it and did replace the O2 senser in the front of the cat about 6 months ago Link to comment Share on other sites More sharing options...
fivepointohman Posted March 25, 2008 Report Share Posted March 25, 2008 That code refers to a rich condition after the cat. It may very well be a bad 02 after the converter, which is a quick fix, but It has been my experience that the CAT is probably shot causing the code. A bad cat, will kill that 02 if its loaded up enough. Link to comment Share on other sites More sharing options...
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