In diesel engines, the fuel and air inside the cylinder is heated by glow plugs. This heat is what causes the diesel fuel to explode, moving the pistons and in turn running the engine. Occasionally, the glow plugs can go bad or malfunction. The main symptom of bad glow plugs is if the engine does not start or has a hard time starting in cold weather. While these symptoms are revealing, it is a good idea to test the glow plugs with a voltage meter -- more commonly called a voltmeter -- before changing the plugs. Fortunately, using a voltmeter is not complicated.
Open the bonnet of your vehicle.
Examine the centre of the engine compartment. The glow plugs are normally lined up in the middle of the engine. They are the size and shape of an average pinky finger.
Examine the glow plug wires. You should see either one wire connecting all the glow plugs or a single wire extending from each individual glow plug.
Switch your voltmeter to ohms. Most voltmeters have several different reading options. For the glow plugs, you need it to be reading ohms.
Place the positive clamp (red) on the connection wire for the glow plug you want to test. If you can't find the correct connection wire, you can also try to place the clamp on the retaining nut at the bottom of the glow plug.
Place the negative (black) clamp on a non painted metal surface in the engine compartment. Make sure that the piece of metal you clamp does not carry a current. This means that it should not be connected or touching the battery or any wires coming from the battery.
Switch the ignition key to the charging position. This allows the glow plugs to "cycle," which just means that they charge.
Read the ohms output on your voltmeter. If the reading lies between 0 and 5, then your glow plug is fine.