A few years ago I had two E-450 box vans 7.3L's from the same moving company towed in two days apart both for a crank no start.
I chased the problem around on the first truck for hours, no codes seemed to have everything it needed to start but wouldn't.
I had the Ah Ha moment when I tried to monitor some pids with NGS because another tech was using the WDS at the time, when cranking the NGS would shut off. Long story short the vans had the setup with one Battery under the hood and one on the frame rail with the big brass nut connecting the battery cables together under the hood. when cranking the B+ would drop to about 8v just enough to shut the pcm off and not start because the big brass nut was a little loose so it was only cranking on the under hood batt. I don't know how long I would have chased that problem if I hadn't had to use the NGS.