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Hi all I just picked myself up a cracking 97 S8 I love the car but she dose have a few issues and I am just wondering if anyone else has been in the same boat and can shed any light as to what it might be. So from cold she fires up perfect and runs perfect until I shut the car off then when I start the car back up when its hot the CAT light comes on and has a missfire and will not go off again. Thoughts are as follows... OBD scan - Got a cheap scanner and came up with missfire cylinder 2 which led me to think coils - So I swapped the 1st and 2nd coils over but the missfire didn't jump (Still might be coils though and I am looking into buying new ones) Ignition amplifier unit - Read online they can go bad randomly I bought a new one as it was cheap enough and already fitted but no differance. Spark plugs - Took them out car has a full set of iridium plugs fitted but they all seem pretty fresh and good (might get another set also to rule this out) My main thought is this when I took the engine covers off to check the coil's etc I found these two plugs on the top on the engine for the knock sensors they are totally borked and fell to bits in my hand Ive ordered two new knock sensors but I also don't know if that would be cause the missfire but they are getting replaced reguardless. Weird thing is I can leave leave the car to cool off or overnight and she will fire right up again no problems plus the car will run perfect so long as I dont shut it off after a run while its hot. Its really got me scratching my head??? But thank you for any input it would be greatly appreciated Cheers Johnny.