It is a 2013 XLT Crew 3.7 with 90K miles. I have been fighting a P354 code off and on for months. Bought the truck in June from dealership and had the code and rough running happen the next day. Took it back and they worked on it and gave it back. It was good for a month the acted up again. Took it back to dealership and since i didn't buy extended warranty they gave me a hard time about fixing it. But they worked on it again and said they put a coil and plug in it, i was told the plug was toast. Also found out that during pre sale inspection they also worked on the issue. This time it didn't have an issue for until a few weeks ago. This time i decided to just take care of it myself. Put new motorcraft coil and iridium plug in. Didn't fix the issue. Swapped coil with #5 and issue stayed at #4. Now I'm lost. Only thing i can think of now is wiring to PCM or perhaps PCM itself. But don't want to just throw parts at it. I would be grateful for any help. Thanks

 

Before diving into the PCM, have you had a chance to inspect the wiring thoroughly? Check for any signs of damage, corrosion, or loose connections. If that doesn't yield results, considering the intermittent nature of the issue, a wiggle test on the wiring while the engine is running might help pinpoint the problem area.
Also, I’d double-check the fuel injector for cylinder #4. A faulty spray pattern or uneven fuel delivery can cause misfires. You can do a fuel injector balance test or use a noid light to check the electrical signals.

 

The P35x DTC is strictly an electrical fault related to the #4. An injector issue won't cause this fault.

The problem pretty much has to be either the wiring/connector or the PCM. Not much else can cause it.

Personally, I'd be inclined to replace the COP pigtail and to run a new wire to the PCM, leaving the old one in place.