Yeah, it was night and day the difference. I do not know if anything got back to the muffler and I am about due to change the plugs, wires, rotor and cap which will probably help even more. I did clean the MAF sensor a couple months ago and even with the CAT rattling if helped some. I was getting about 13.5 mpg (mostly city driving), cleaned MAF and it improved to 14 and still running the first tank after the new CAT. Will let everyone know what the improvement is from the new CAT soon as I have to fill up again.

 

First if coolant, oil ect.. gets to the cat it could cause a small explosion in the honey comb of the convertor then it breaks up inside if the pieces are small or the contamination continues it may break up small enough to clog muffler and cause to much back pressure and truck will ide fine but may not be able to run above that. a oil clogged converter will do the same, if you take exhaust loose at y pipe it runs fine.
then some times like my converter the honeycomb is held tight in its shell by a fiberglass pad on the outside, something causes this to burn away mine was coolant leak in manifold, this lets the honeycomb rattle around about a 1/8 of an inch in its shell when at idle.
the converter like this works just fine just rattles passes all the on board monitors in computer but sooner or later it will start to breakup but I'm gonna run mine like it is till then.
may get weeks or years out of it, but since it sill works I'm gonna keep the cash in my pocket till it completely fails may need the whole exhaust don by then when the tail and muffle need replaced ( I'm not a penny pincher but why replace it if it sill works I can live with the rattle I don't hear it in the truck 90% of the time.)

Just crank up the tunes an ignore the strange looks from the car next to you!!!!

 

That explains a lot, I actually had coolant leak that started several years ago and I did not get it fixed right away (I think it was the intake gaskets that were leaking coolant). But shortly after I noticed the coolant leak is when the cat first started rattling. So after the leak was fixed I let it go because I had another car and the Blazer was just used on weekends or for moving stuff or in the snow for the 4WD. But recently the rattle kept getting worse and I was noticing a performance drop. So yes, you can let it go as long as it does not plug up the cat too much and if it throws the catalytic efficiency code then its probably time to replace. Mine actually stopped rattling for a while and then threw the code, but the rattle came back and the check engine light went off. But I think most people would recommend not waiting that long from what I have read because you could end up having to replace the muffler as well at the very least.

Correct me if I am wrong, or update with other problems it could cause. Thank you to everyone for all of your comments and information, this forum has been a great learning experience for me with my Blazer.

 

Hell i'll throw a little more insight in this old thread. Why not. I just replaced my LIM gaskets a few weeks ago. They must have been bad for a while (PO was my stepfather and never did find that coolant leak.....I DID)....anywho....my cat started rattling pretty bad shortly after. Yet another case of the IM gasket wiping one out it seems.