Pull the cover off and wiggle the connections. You'll probably find that a wiring harness isn't making proper contact, and a little poking around should at least help you troubleshoot. Mine works on and off. If I give it a good smack it usually comes on! Been meaning to do a more permanent fix, but just haven't gotten around to it.
2002 F-250 Lariat CCSB 7.3 PSD; PHP Phoenix (TS 6 position), DIY 6637, foil delete, Banks exhaust brake, air bags | 22x,xxx and running like new on original injectors and hpop