The valve cover gaskets may leak oil into the spark plug wells, shorting out the spark plugs, causing a misfire, and illuminating the Check Engine Light. Commonly the spark plugs and ignition coils or wires will require replacement, along with resealing the valve cover to correct this concern.
Problems for specific Ford Thunderbird years:
Car problem reports
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Most reported 2004 Ford Thunderbird problems
There may be an extended warranty on the ignition coils - see your Ford Dealer for more information-Thanks John B for this information!