I have a 2006 6.0 dually. For 13 years it has been great.
Just recently it has started having issues. I replaced 2 fuel injectors that were reported to be "missing".
It ran fine for a few months. Then I rebuilt the fcim and again it ran good.
Then it started a new issue. After starting and running for about 20 minutes it dies. After letting it cool down for about 30 minutes it starts and repeats the process. Everything I read says its the IPR valve. So I ordered one and replace it just like on YouTube.
Now it cranks but won't start. Batteries, fuel filters, oil filter, air filter, IPR valve, FCIM are all new. Oil level is reading normal.

What now? Please help I will consider all ideas helpful.

Get a monitoring device that can read engine parameters. Then monitor the high pressure oil system when cranking (ICP pressure, ICP volts, IPR % duty cycle).

It might be the IPR valve if it is aftermarket (lots of bad experience w/ aftermarket sensors), but the dummy plugs and standpipes are prime candidates for problems as is the HPOP STC discharge fitting if you still have it.

Are you getting the oil pressure to register on the dash when cranking?

I replaced the dummy plug and stand pipes. They are torqued to specs.

What did the O-rings look like? Specially, on the dummy plugs? That will tell you if that was the source of your HPO leak.

They looked great and i don't think I have an oil leak. I ordered a scanner . When it comes I will post the numbers.

You can't see high pressure oil leaks. They will leak from the high-pressure system back into the crankcase.