I've always known a clogged cat on late model vehicles to throw a code, however is this always the case? Will it always trip a code if something is wrong with them? I have a friend here at work today asking me and I didn't want to give an answer not knowing fully.
04 silverado with the 5.3.
Thanks Gentlemen.

 

It *should*, but sometimes strange things happen. I've seen cats clog, and cause misfires and no power above 4k, and the only codes that were thrown were multiple misfire codes...

 

Been working on cars for 20 yrs and hav'nt seen an ses light help diagnose a clogged cat...whats the truck doing? easiest way to diagnose a clogged cat is drop the y-pipe and drive it...i assume its down on power, check codes, fuel pressure and volume, drop the y-pipe, un-plug the maf...that should get you started.

 

...or you can hook up a vacuum gauge and see how it reacts. If you rev the engine up above 2500 and snap the throttle shut, the vacuum gauge should quickly jump up, usually over 20 mm/hg. If it slowly creeps, there is a restriction in the exhaust. A backpressure tester in the suspected bank's front O2 sensor bung will also tell you if the exhaust is restricted.

 

I had p0300 multiple misfires and shitty power up top.......