No start No crank

Make sure battery has a full charge and battery cables/wires all ok, on both ends, not just at the battery. Make sure tranny is in park or neutral and parking brake is set. With intermittent issues, you have to test when problem is ongoing. I can only help with testing. When starter won't crank, pull the starter relay in underhood fuse block. Terminal 30 is hot all the time. Use proper jumper wire between terminal 30 and terminal 87, the starter motor should crank the engine. Again, make sure tranny is out of gear and parking brake is set. If it cranks then hopefully the starter motor is ok. That might show the problem is on the control side of relay. I'm not saying the relay is faulty. The crank fuse should be hot with key in the crank position, you can use a test lite and check that. I could go on, but, you need to do some testing. Intermittent issues can be a pain to track. I don't know what testing your mechanic performed. Unless the mechanic can get the vehicle to act up when he is testing, he won't find anything. I couldn't either.

Put brand new battery, changed relays starter has power. Never had problems till I had mechanic fix flywheel because it was making a clacking embarrassing noise. Never had problems with it starting. I get it back then it next day no start just power. Hours later it started. I'd go places didnt have have starting problems that day. Next day nothing. I told mechanic he said he fixed a wire. I get it back next day nothing. Its been parked for a week now it wont starr or attempt to start just power from everything else. I Changed the relays. Still nothing. Mechanic said he can't get to my truck till middle of next week.

Did u Everett find out the real problem,I'm dealing with the same shit

I had an issue with a shop putting the wrong flywheel on my 6.0 . I’m having a similar problem right now that I think is related to the dumb security system factory installed

Try the camshaft position sensor..if it or the crank sensor is bad it won't start,just turn over

I had that problem too, when I changed my transmission, it wouldn’t turn over, solenoid wouldn’t snap or engage even. Turned out to be when I removed the starter there are 3 terminal nuts, the big one is power to the starter, the little one is feeding the solenoid from ignition switch, the 3rd one is a ground strap which I was having trouble loosening because the check oil tube is in the way so looking for the other end realized it is actually grounded right to the starter itself and doesn’t need to be removed. Turned out I actually did loosen it a bit ( didn’t think I did ) but after removing again to check it out, it was barely loose and just not making contact. Tighten it up. Put back on and good to go. If ground is not tight it will not work, I kinda thought with the 2 bolts holding it to the block it would be grounded anyway, but not the case. Hope this might help someone else out.