Having problems with coil F (6) throwing a check engine light. Been repaired by Ford with a new coil. 2 weeks later threw another code same coil F (6). Went back they said plugs were loose and needed tightened, so they re tightened and sent me on my way. 1 month later threw another check engine light same coil F (6).
For the time being I switched coil 6 with coil 5 to see if I got a code from 5. Then I could rule it as a bad coil. Drove over 200 miles without an issue as of right now.
Could be a bad connection in the COP wiring harness plug. Also, be aware, I replaced all 8 of mine with Motorcraft COP's one year ago and have just replaced the fifth bad one out of the batch. The bad thing is that I just replaced them last year as a preventative maintenance measure. So it could be a problem with the replacement COP too.