My 2004 6.0 L is setting all 8 glow plug/heater codes (P0671 thru P0678).
What would cause this condition? No other codes are showing - not even the glow plug control module. It starts up very easily, but the outside temp is in the 70's.
Thanks for any helpful advice, Joe

 

I would say gpcm, that is what happend to me awhile back. but you can ohm test the glow plugs to see if they are working

 

It may be a harness issue as well. If the signal can't get from the module to the plugs they won't work either. Corrrosion in the plugs maybe.