3.5 Ecoboost, CEL and misfire
 

2014 3.5L Ecoboost
Last week I was accelerating on the interstate and the check engine light starts flashing and I can feel the engine start to shudder. This is my 3rd F150 with the COPs and I pretty much knew it was a bad coil pack. I go straight to Twilight Zone and get the code read. It's P0303, misfire in cylinder 3. I buy a new coil pack and put it in and the misfire is gone, engine is running smooth again. I drive it a couple days and the CEL light is still on. I stop by the Zone again and get the code read, still P0303. I buy a new Motorcraft plug and put it in. The one I took out didn't look that bad. Drove to work this morning (about 25 miles) and the CEL is still on.

Does something need to be reset? I did disconnect the battery yesterday when I changed the plug. The guy at twilight zone said it could take 50 miles of driving before it goes off. I read somewhere else that it could take 10-20 driving cycles for it to go away. There is no noticeable misfire than I cat tell, the engine sounds normal, no shuddering, I am at a loss as to what is going on.

Also, I read several threads where people recommended changing the plugs at 40,000 miles. I am probably pushing 45k on the current set. Should I go ahead and change them out?

I would change all the plugs. What brand coil did you use?

I used the Duralast house brand at Auto Zone since I was there and I needed a coil. I replaced all of the coils about two years and 45k miles ago along with all new Motorcraft plugs. The other 5 coils are United Motor Products that I got on Rock Auto.

The only coils I recommend are Motorcraft, Denso, or Delphi. Anything else can be a crapshoot. (Note - Denso is the Motorcraft OEM)