Diesel glow plugs heat up the diesel fuel in an internal combustion engine so that the engine will run properly. When the glow plugs are not functioning properly, you can replace them, but you should test them first to make sure that the plugs are in fact defective before replacing them. For this, you'll need a voltmeter.
Set the dial on your voltmeter to "ohms."
Touch the red lead on the voltmeter to the tip of the electrode.
Touch the black lead to the glow plug's retaining nut.
Check the resistance flowing through the glow plug. The resistance should read between 0 and 5 ohms. If it is greater than 5 ohms, the plug has failed and needs to be replaced.