Glow plugs are installed on many diesel engines to help with cold starts. They usually fail because of corrosion, overheating, mechanical damage, or metal fatigue, and their failure can cause a variety of problems. The easiest way to test a glow plug is by using a clamp-meter, though digital multimeters can also do the job, and glow-plug testers also work. We will cover the following topics in this post: What is a Glow Plug? What are the Symptoms of a Failing Glow Plug? How to Test Glow Plugs with a Clamp-Meter How to Test Glow Plugs with a Digital Multimeter How to Test Glow Plugs with a Glow Plug Tester Caution: Be careful to follow the glow plug manufacturer’s precautions when testing, as well as the test tool’s directions/manual, and read the vehicle (or other system) manual before doing any test.