How do I test the core over spark plug units to find bad one?

FIRST SCAN CODES TO GET MISFIRE CYL AND THE SWITCH COIL TO ANOTHER CYL AND SEE IF MISS FOLLOWS COIL.THEN THAT COIL IS BAD EASIEST WAY TO TEST

Take it to the dealer. There is an extended warranty on the coil on plug for the 2004 thunderbirds. It's 10 years or 100,000 miles. Ref: program 07M07 and Program 07N09. I received my letter dated June 2008. Good luck.

How many miles are on the car? There is a little known warr. from Ford that covers those ign. coils for 100K miles. They will drop a code when they fail. A differant code for each coil. And in most states you can declare the car a lemon under the Lemon Law.