My 2008 4.2 has 250kkm on it. Things do need to be replaced. Some because of wear / tear or others because they break / broke. I was wondering about the cat. Do they wear out suddenly... no not clog but just stop doing what they are supposed to do ("clean gases")? I recently got a endoscope to stick up its "nose". Although they thing is not looking clogged... maybe some part around one area, I was wondering about the general aging. I am assuming the O2 sensor and monitoring system have some way of determining that it has tipped over but I guess the question is more will it "limp into old age" until it gets either clogged and cracked inside? I am hoping that I never have to deal with this although getting a "thumb quote" from a local muffler shop ($1100 for the part... ya right... :-0 ) will certainly put a lot of pressure on my wallet should things go south. The same shop didn't appear to know what I meant by a "back pressure" test sheesh! But that is kind of expected here in the great white north... at least with my experiences with shops.