Hmm... Never heard that the sensors are going to be too sensitive to high flow cats on the 3800. GM's cats are pretty efficient on this engine and the gains from high flows are minimal so the sensors should be fine. If you are rattling inside the cat then yes get a new one. If MD is plugging a device into the OBDII port they are getting your emissions readings too. The OBDII port can not only deliver codes, be used as a programing port but it also gives out the emissions data so they dont have to put the sniffer up the tailpipe. Keep a cat in there and you may want to see about changing the rear sensor after replacing it.

Massey

 

It is not a problem with OBDI cars because OBDI do not use downstream sensors, and OBDI does not look for one.

 

I think I am going to try a high flow cat and see what happens.