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Well, the spark plugs being worn out could have caused the misfires, it would have been pretty obvious to you that the engine was misfiring if it was bad enough to ruin a cat though. The info you gave is that you had a check engine light, and AFTER the plugs were replaced THEN the engine ran badly, not BEFORE, or did it run badly before replacing the plugs?
The problem as I see it isn't that you had a check engine light which they blamed on misfires and they replaced the plugs, what bothers me is that they are saying the cats are now ruined, but the truck only ran rough after they replaced the plugs.
Gilly
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