O.K., so the muffler shop pressure tests the cats and confirms that they are plugged. They take some measurements & #'s off the cats so they can try to match them up with a replacement... So I think to myself, great finally going to get somewhere on this problem... 2 hours later I go to start the Ram & major mis-fire... running on 7 maybe 6 cylinders, CE light comes on. When I get it home, I read the codes stored & it tells me there's a missfire on #8... so I slap a timing light on it & it looks o.k. and it makes me wonder... how does the ECU know it's the # 8 cylinder? Is it monitoring the RPM & cylinder timing to determine which cylinder is a problem?

I didn't have much time to go through it but the #8 cylinder is the closest to the cat that's plugged the most (690 deg), with this much heat is it possible to damage a valve on that cylinder?