Glow plug control modules are generally irreparable and should be replaced when faulty. Symptoms of a problematic GPCM include hard starts, no start, engine misfires and a P0683 code on the check engine light. If you notice these problems in your diesel engine vehicle, then the GPCM is one of the components that needs to be inspected.
To test the GPCM, make sure the engine is completely cool. Open the hood and test the GPCM using a multimeter. To use the multimeter, connect the positive lead to the GPCM post and the negative lead to the ground point, which is the battery's negative terminal. Set the multimeter to DC volts, 15-volt scale.
Insert the key into the ignition and turn it to the "on" position. Do not turn the key all the way, as the purpose is to cause the glow plug relay to warm the glow plugs. As the glow plug warms, the multimeter activates. If the GPCM is operational, the multimeter should give a reading between 10 and 15 volts. If the multimeter does not register any voltage, it may be time to replace the GPCM.
To confirm if the GPCM is indeed the problem, check to see if there is voltage going to the GPCM. If there is, replace the GPCM, but if not, then the problem is likely in the powertrain control module.