Troubleshooting Common Glow Plug Problems

Troubleshooting glow plug problems in your diesel engine is needed if the car refuses to start and has become low on pick up. Here are a few common glow plug problems and methods of troubleshooting them.

Check Parts

See which parts of the engine glow plug are easiest to access. Start with those that are the easiest to check and gradually move up to the more difficult or complicated parts. By working in this order you can perform the tests more easily and systematically.


Check the voltage of the engine glow plug. On turning the ignition to pre glow setting, the voltage reading must be positive for a few seconds at least. On getting a negative reading, check at the body of the car. Sometimes the reading fluctuates wildly but it is nothing to worry about. However, if there is no reading registered at the fuse, you need to repair the relay in the plug. In case the relay has voltage, check your battery.


Test the engine glow plug for resistance. This must be taken after removing the strips on them. A reading that is less than 1 ohm is normal. All glow plugs must have uniform resistance. In case, any plug shows a high resistance reading, that must be replaced with a new one.

Warm Plugs

A simple way to check for bad glow plugs is to cycle glow plugs in the car thrice during the morning. The cylinder head shall heat up and become warm. In case you see that any part around the glow plug is not warm, those are the ones that need replacement.


Locate the wire connected to the temperature relay of the engine glow plug. It might have been disconnected and this might be the problem. Re-connect it in such a case. See if the glow plugs are receiving the required amount of power supply, by touching a tester to the plug. The tester bulb lighting up shows that it is in perfect condition.


Check that the plug relay is able to give off the power needed for starting the engine by checking the resistance of the circuit using the ohmmeter. A relay that is in working condition must show a reading between 50 to 120 ohms. A reading of less than 50 ohms is bad news.

Plug Pins:

Taking the relay socket, test each pin. Each must have a power supply and voltage, 30 needs to show battery voltage, while 50 should have power while the engine runs. Battery voltage must be seen in 85 and 86 must show power while the key is at the slot for pre glow. 87 must have direct power to glow plugs. The L part should show that there is power supplied to the LED of the dashboard. The plug temperature sender must be getting input from the head of the cylinder. If any of the conditions mentioned, differ in any part, the relay needs repair.

Wire Connections:

Connect a wire starting from the plug to the dashboard LED. If a click is heard and the light flickers on and then, off despite there being no power supply to the engine glow plug, the relay is surely not working properly. Make sure the plug’s fusible link is checked for nuts or wires that may have become loose. A loose part often causes the trouble in the relay.