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The GMC Truck glow plug heats the diesel pre chamber up to a temperature so that the diesel fuel when injected will ignite. If the GMC Truck glow plug or glow plugs are bad, the diesel powered engine may refuse or at a minimum is very hard to start. The only way to test the GMC Truck glow plugs short of removing them is to use an Ohm meter and check the resistance of the glow plug.