Repeat code P0304

[dsedlow]

2008 Ford Expedition 5.4, currently 121,000 miles. Vehicle had P0300 and was taken to a shop, they replaced #1 and #7 coil and told owner that the spark plugs needed replaced but that the heads would probably have to come off because of the Triton plug breakage issue. I replaced the spark plugs and engine ran perfect for 4,500 miles. The owner started feeling misfires again, code p0304 this time, I exchanged #1 and #4 coils, test drove vehicle, code then p0301. Replaced coil. Drove vehicle about 100 miles, now misfiring again code p0304. The coil currently on #4 is from #1 and only has about 5,000 miles on it. Did it mysteriously fail or is there some other ghost in the engine? Thank you.

 

[2007EXPYBLUEEB]

I had similar continuous misfires codes. CEL flashing at times.
Eventually I got cat conv codes-driver side.
Not sure if the unburned fuel ruined the cat, or if a bad cat fouled the plugs, as I was getting multi cylinder misfires.
I replaced the converter and then the plugs/coils, which remedied the issue.