One or more glow plugs are inoperable. Maybe from wiring or connector issues. Or possible bad glowplug(s). If you turn the key on without turning the motor over and let it set. The glowplug light will come on, but it takes about 1-2 minutes. This is to tell you that there is an issue in the glowplug circuit.

__________________
2005 Dodge Ram Big Horn Hemi

2011 Honda Insight Lx

1994 BMW 530i Touring Wagon