How to Troubleshoot a Glow Plug

Updated February 21, 2017

A glow plug is a device used in a diesel engine. Its function is similar to that of a spark plug in a standard car engine. These plugs are heated, which in turn heats the compressed air in the combustion chamber to move the engine's pistons. When then engine is not starting or is running rough, there may be an issue with one or more of the glow plugs.

Move the vehicle to a garage or other covered area and then pull the hood-release lever. Open the bonnet and set the support bar.

Locate the glow plugs that are near the centre of the engine compartment.

Verify that the glow plug temperature relay is connected. Re-connect the relay if it is loose or has become disconnected.

Verify that the electrical wires are connected to the power relay. The power relay is the black plastic box on the driver's side of the engine, near the wheel well.

Remove each glow plug with the socket wrench. Examine the tip of the plug to see if it is dirty. Clean a dirty glow plug with a rag or cloth.

Tighten any loose glow plugs with the socket wrench.

Start the engine and test each glow plug with a voltmeter. The normal voltage for a glow plug is 0.8 to 1.4 volts.


The vehicle needs to be turned off before working with the glow plugs. Glow plugs should be replaced in a set instead of one at a time.

Things You'll Need

  • Socket wrench
  • Cloth or rag
  • Voltmeter
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About the Author

Cameron Easey has over 15 years customer service experience, with eight of those years in the insurance industry. He has earned various designations from organizations like the Insurance Institute of America and LOMA. Easey earned his Bachelor of Arts degree in political science and history from Western Michigan University.