Clogged cat ?

I just replaced my cat (98 Chevy truck 165,000 miles) for the same reason. Mine has the cat & Y-pipe as one unit ($500). I took it off & gutted it out to stop the rattle, which resulted in an error code due to two O2's before & one after the cat, so a "Test pipe" will not work, unless you replace the rear O2 sensor w/ a chip that Jegs sells for around $80. Tuffy Muffler welded in a replacement for $120. Solved my rattle & error codes, but I may still have a clogged muffler.