How to Check Diesel Glow Plugs

Updated February 21, 2017

Diesel engines are equipped with special plugs, called glow plugs. The glow plugs assist in the igniting of the diesel engine. They are classified as a heating device as they preheat the engine and allow other components to warm up, making the engine more efficient. Glow plugs can become worn out and defective. When this happens, the diesel engine does not preheat properly and can cause the engine to run roughly or become damaged. If you suspect your glow plugs are defective, testing them with a volmeter, which will test the amount of heat the plug emits, is the best way to determine if they need to be replaced.

Place the vehicle in park and allow the vehicle to cool down for twenty minutes. This will prevent the components from being too hot to touch and causing burns.

Open the bonnet of the vehicle and locate valve cover. Use a socket wrench to remove each bolt holding the valve cover in place. Remove the valve cover to gain access to the glow plugs.

Remove the electrical connector, using a pair of pliers, from the glow plugs. Remove each glow plug using a socket wrench.

Turn the volmeter on and switch the settings to the "ohms." Lightly touch the electrodes on the volmeter to the glow plug. If the glow plug shows a resistance reading less than 5 ohms, the plug is not defective. If the glow plug shows a resistance reading more than 5 ohms, the glow plug needs to be replaced. Repeat this process for each glow plug.

Things You'll Need

  • Socket wrench
  • Pliers
  • Volmeter
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About the Author

Johnathan Cronk is a freelance writer and began writing at the age of 18. Throughout his career he has specialized in sports, how-to and advice articles. He has also written sales pitches in the corporate setting since 2001. He studied business at Hudson Valley Community College before transferring to the State University of New York, Albany.