Since diesel engines are essentially self-igniting because of the low burning point of diesel fuel, they operate without spark plugs. Glow plugs are designed to help with the starting of diesel engines when temperatures are very low. Glow plugs are able to quickly warm the area near the top of the cylinder head by means of an electrical current that flows through a heating coil at the end of the glow plug. Glow plugs will not last the entire lifetime of an engine, so they should be tested to make sure they are still working properly.
Disconnect the battery of the vehicle to prevent electrical shock.
Disconnect the electrical wire that is connected to the glow plug.
Connect one of the leads from an electrical test light to the positive terminal on the vehicle's battery.
Touch the other lead from the electrical test light to the terminal on the glow plug being tested. If the test light comes on, the glow plug is good. No light means that the glow plug needs replacing.
Repeat the process for all the glow plugs in the engine. Replace the entire set if any of them test bad, as the others usually fail soon afterward.