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A single glow plug will not cause this truck to not start. If it ran before you removed the tune than my guess is something went wrong and the processor may have been erased or the stock program is corupt try reinstalling a program and see if the truck will start. If that is not it check and make sure there is fuel at the top fuel filter with the key on.
More Common causes of a crank hard start / no start will most likely be a internal oil leak at the STC bracket or dummy plugs or a bad IPR (Injector Pressure regulator), ICP Injection control pressure sensor or the FICM modules go bad and will throw 8-10 injector codes eventually causing a no start concern. If your ICP is located in the rear of the engine on the high pressure pump those tend to leak oil. This causes various problems and the ICP needs to be replaces along with the connector. If the ICP is in the rear it doesn't have the STC or dummy plugs and chances are it’s just the ICP gone bad or IPR. The later 6.0L will have the ICP in the top front of the passenger side valve cover. This generation 6.0L has the STC and dummy plug problems.
I hope my assistance helped you and resolved your concern, if you need further assistance please ask.
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