Testing for locked up A/C compressor
 

I have an 03 6.0 Drove just fine, turned it off, went out to start it the next morning and was an extremely slow crank and it wouldn't start. I put new batteries in, didn't help. Put in a new starter and that didn't do it either. I believe I have a locked compressor. It doesn't spin. I have seen people saying theres a way to disengage that to start it and if it starts that was the problem. I'm looking for steps on how to do that and what I need to do. Thanks for any help!

Remove the serpentine belt and crank. Verify it spins properly then. If you don't want it to start when cranking that way, remove the FICM relay.

W/ the belt off, try rotating the compressor by hand.