I got check engine light on my 08 tc and when I called onstar they said misfire in spark plug 3.So I take it to the dealer and after checking it for awhile they said the plug was fouled and they replaced it.The thing is 4500 miles on the car always used 93 octane and the service writer told me to try the lower octane like 91.I drive mostly highway(try to keep it below 70mph dont need more tickets) but I do also sit in some traffic on commute home too.So is this guy full of it or does he have a point? basically I am not driving the car fast and hard enough (never thought I would hear this from a dealer service writer)

 

Well, I have an 08' TC aswell and that was one of my problems. Cept I had multiple problems with it too, like evap canister, ecm, cruise control..... All is fixed except for cruise control.......but if it was just that spark plug then it was probably a faulty one. Drive like you normally do, but not too crazy and if it messes up again take it back and they will have to investigate deeper into the problem...