Ok I'll start from the beginning, I bought a 2003 Yukon it started throwing a misfire code so I put new plugs an wires it ran great but soon started acting up once again. No power after 3000rpm, then it started knocking really bad at first start then a lite tap to nothing. Now it's throwing a catalyst efficiency low on bank 1 an a random misfire. Is this my cats gone bad? An the knock, sometimes it's worse than others, I can rev it to about a grand while it's knocking an it goes away. Any ideas? We just had a little boy an a girl don't really want my wife in something thatsabout to blow, thanks guys

 

The sound may be common piston slap and the loss of power could just be a clogged cat. Depending on the mileage and the date the truck entered service the cat could still be under warranty so you should call your local dealer if your under 80k miles if I remember correctly.

BTW what plugs did you put in it?

 

It's got 150k on it, my silverado has piston slap an it sounds the same but seems louder. I put ngk's in it. Same brand of plugs I'm running in my cammed 01 without any problems

 

How does the exhaust smell???

 

Idk, I try not to make a habit of sniffing the exhaust lol

 

Well if it smells like roten egg's thats a sign of the cat(s) going out...
Has any codes popped up???

 

My 95 LT1 firebird did that... Around 3000rpm it would cut out... Cat was melted and clogged... I beat all the stuff out and cleaned it out with a shop vac... Didn't have emissions and it ran 100% better after... See if you can try that or see if it was melted like I did.

 

Knocking really isn't that bad today but it's skipping bad after 2500 rpm range, I mean if u pulled out in traffic an had to move it you'd be in trouble