Age and it can be accelerated by other conditions like running rich, short trips where proper operating temps aren't attained for sustained intervals. Exhaust leaks can also shorten the life of the cat. ALL cats will fail at some point but these modern cats, under optimum running conditions can last 200,000+ miles...

 

Good info, thanks hammer. I'm not sure I suffer from the same problem as the op, but I think i will look into changing out my egr valve. My plugs have about 5k on em, my throttle body was cleaned same time as the plugs, I also checked the pcv at that time, it works fine and I cleaned it up. Also I just put a new cai system on, so my filter should be perfectly clean. The truck has done it ever since I bought it about a year ago so I figured it was normal, and it's not so bad that it ever really concerned me much. If the egr fixes it I will report back.