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How to Test Glow Plugs

Glow plugs ensure your diesel car starts – no matter how cold it is. Located in every cylinder of your engine, they ensure your car burns fuel efficiently, with minimal polluting gases. But, what can you do if you start to suspect that your glow plugs aren’t working as well as they should be?

How can you tell if your glow plugs need to be tested?

There are three main signs that show you that you need to test, and possibly replace, your glow plugs:

  1. White exhaust smoke 
    If one or more of your glow plugs is defective, it will allow diesel to leak into the exhaust pipe. As the exhaust heats up, the leaked fuel will burn and produce white exhaust smoke. This will only last for a few minutes after you start the engine.
  2. Hard to start 
    While you can still drive with one broken glow plug, you’ll find it extremely difficult to start your car as soon as two or more glow plugs malfunction even if the weather is warm. It’s important to note that a problem starting your car can also be a symptom of a problem in the fuel system or battery.
  3. Poor engine power 
    As well as making it difficult to start, bad glow plugs will make it difficult for your car to run smoothly due to the improper starting combustion which reduced engine power and efficiency.

How to test glow plugs

It’s important to use the correct tool to test glow plugs as using the wrong tool can damage your glow plugs and ignition system. We recommend the glow plug tester as it tests glow plugs quickly and easily, giving clear results every time, regardless of the voltage your system uses.

Materials needed

  • Glow plug tester
  • Basic set of hand tools
  • Flashlight
  • Owners’ manual

1. Disconnect the glow plug

Most glow plugs are installed in the cylinder heads and will be connected to a heavy gauge wire. Remove any covers that may be obstructing access to the glow plug. Then disconnect the car battery and the electrical wires connected to the glow plug to prevent electric shock.


2. Test your glow plugs

Connect the tester to the battery with both the (black) negative lead and the (red) positive one. Then connect the blue cable to the glow plug. The tester subjects the glow plug to a 10 second test under real-life conditions.


3. Test results

The results are clear. If the glow plug is working as it should be, the ‘OK’ symbol and the dial are lit up with a green light. However, if the background light is red, the glow plug is defective and should be replaced.


4. Repeat and replace (if necessary)

Repeat the test for the other glow plugs in the engine. If one glow plug needs to be replaced, it’s recommended to change them all. And, remember to test all new glow plugs before you install them.

If you want to learn more about testing glow plugs, watch our video: 


For informational purposes only. We are not liable for any damages resulting from your reliance on this content.