This is the exact same thing that has been ocurring with mine. If I cruise my Caddy it throws the code but when I give it some good ole lead foot it doesn't. that is why I have not pull the trigger on a Cat, yet. But as soon as all this snow clears I will buy one.I have intermittent P0420 in my Deville and it comes only when I drive easy. Normally I drive hard so I never see it. The recent day that I got the code & SES was last week, when it snowed and everyone had to drive very carefully. Do WOT runs. Worth a try.
Sub could not have said it better...I had the same problem on my 2001 DTS after doing the plenum boot and IT gaskets. I did the WOT and BINGO! No more SES No more P0420 code! Passed emissions here in Ohio today after work!BEFORE you lay out long green for cat work, go up to the Cadillac Technical Archive ^^^ in the black bar and read it all - especially the article "Occasional full throttle operation is good for your engine.......". This procedure will clean out a sluggish exhaust system and allow the O2 sensors to perform their efficiency comparisons.