How to Know When Glow Plugs on a Diesel Are Bad

Updated February 21, 2017

Gasoline-powered engines need a spark to ignite the fuel to cause combustion, thus moving the pistons and getting the engine running. Diesel engines, however, rely on compression to start the engine, so instead of spark plugs, the diesel engine requires glow plugs. The glow plugs begin burning the fuel before the compression cycle begins, and help the vehicle start at a low temperature. Every now and then a glow plug becomes defective from use. In this case the bulb has typically burnt out. If you suspect that your glow plugs are defective, a testing method can be used to check if the glow pugs on a diesel have malfunctioned.

Place the voltmeter clip-on current clamp onto the glow plug supply wire. This is a bell-shaped wire on top of a single glow plug. The placement of glow plugs in a diesel engine will vary from vehicle to vehicle, consult your owner's manual if you're having trouble finding them.

Turn the key and flip the ignition switch to the warm-up position. This will prime or light the glow plugs.

Look at the reading on the voltmeter. A defective glow plug will read less than 15 amps. A properly functioning glow plug will read 15 to 20 amps.


If you try to start your engine and you hear it cranking over but it fails to start, and this is followed by a plume of smoke emitting from your exhaust, then you may have one or more defective glow plugs.

Things You'll Need

  • Voltmeter with clip-on current clamp
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