The Chevrolet Caprice glow plug heats the diesel pre chamber up to a temperature so that the diesel fuel when injected will ignite. If the Chevrolet Caprice 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 Chevrolet Caprice glow plugs short of removing them is to use an Ohm meter and check the resistance of the glow plug.