'04 5.9 always starts cold with a little cranking. Occasionally won't start when warm (after normal driving). I am guessing it could be a high pressure fuel problem as I have been told that electronics won't let engine start until it builds high fuel pressure on rail. Potential problems could be the high pressure limiting valve (relief valve) on the rail or an injector, not sealing and not letting the rail build pressure, but those I have talked with about the problem seem to think it should be worse on cold start if that was the problem. I replaced the fuel filter and the fuel lift pump and nothing changed. Otherwise truck runs great. 250+K miles. Let me know if anyone has had same problem or has other input. Thanks

 

Leaking injector, you can try capping each injector at rail one at a time until you find one causing no start. You need this capping tool.
https://www.handsontools.com/Miller-...UaAroHEALw_wcB