I have a code on my 2005 F250 6.0 diesel that shows all 8 glow plugs open. I replaced the glow plug relay but that didn't fix issue. any suggestions would help. Cant imagine all 8 went bad at same time. Codes are P0671 thru P0678

 

What you might try is to start at the glow plug relay and confirm power is there and going through when the relay is energized. You need to check each side of the engine to confirm that power is getting to the glow plugs. A 12volt test lite usually does the job.
What you are looking for is the place where the power is lost on the way to the plugs.

 

Check the ficm to make sure all the connectors are plugged in all the way.