Unlike a gasoline engine that needs a spark to catch fire the fuel, diesel motors are based on extremely high compression. Diesel compresses the air inside the cylinders until the air reaches high enough temperatures to easily ignite the fuel. But if a diesel engine block is cold, very little heat is created through compression to ignite the fuel.
If the Powertrain Control Module (PCM) detects that the engine is cold through oil signals and transmission temperature sensors, it commands the glow plugs to ignite.
Basically, the diagnostic code P0683 OBDII means that communication between the Glow Plug Control Module (GPCM) and the Powertrain Control Module (PCM) circuit has been lost. The fault prevents the PCM from commanding the Glow Plug Control Module. This command is important as it is an on/off signal.