Hi John,
when you changed spark plugs you did go back with NGK plugs the same as what was in there i hope.
the plug gap is also supposed to be.035 (.09mm)

You may also want to swap out the ECM relays in the relay box, they have been known to cause issues and make the truck throw codes.

I did find the F04 is an ECM communication error code (most of the time related to the relays)
The P04 is the same as the F04

I did have some information on what the prefix code means in relation to the truck and what system it was associated with. If i can find it i will return and post that here, it is very helpful in giving an understanding on where to look when you see the P or E or F or L or even D prefix. That has always been a headache with mfca when it comes to relaying information, you never know exactly where to find it and spend loads of time looking... its terribly inefficient imo.