Using a Multi-Meter to Check Glowplugs
- Ryan Brown
- Oct 14, 2025
- 7 min read
Today, we explore Glow Plugs and how a test meter can indicate whether a glow plug is faulty before you replace it.
What are Glow Plugs?
Glow plugs are heating elements, used to bring a diesel engine's combustion chamber to the ideal starting temperature. This aids starting in cold weather and helps with fuel efficiency.
Diesel and Temperature
Diesel fuel requires high compression and heat in order to combust. The pressure is produced when the engine spins, by pistons compressing the air-fuel mixture within the combustion chamber. Though diesel engines require some level of heat, they don't need as much as petrol engines, which use the high-intensity heat from a spark plug's arc to ignite.
However, when an engine is cold, one of these primary necessities is missing, which can hinder the starting process. This is why some diesel vehicles may have trouble starting in freezing temperatures, even with a healthy starter motor and a charged battery. This is especially true if there are issues with the glow plug system, preventing the elements from adequately heating.
Glow Plugs in Action
Glow Plugs work by heating up a tip that protrudes into the engine's combustion chambers. They are generally controlled by a Glow Plug Relay, which provides a high current supply to the glow plug anode terminal. The plug is grounded through its outer body, which is bolted into the engine block.
When you first turn on your ignition, the glow plug control relay switches, energising the glow plug elements. The temperature from the element directly heats the compressed air-fuel mixture inside the chamber. This exposure to heat brings the fuel to the ideal temperature for combustion, even if the engine itself is cold.
This heating process is typically timed so the elements are energised long enough to heat the chamber, but not too long that they burn out. Once the engine has started, it produces its own heat, and the glow plugs are no longer needed.
Common Symptoms
Modern cars can typically detect if there is a break in a glow plug circuit, illuminating a fault light on the instrument cluster or displaying a warning message on a screen. However, issues with the glow plugs can still hinder performance even if they are not completely faulty.
If the glow plugs do not turn on or do not heat enough, this can limit the initial injection of heat entering the engine. This can lead to cars starting poorly, especially in cold weather.
Additionally, even if one or two glow plugs are performing poorly, this can lead to the engine burning fuel badly, causing an excess of soot or smoke until the engine warms up.
As mentioned previously, once an engine has started, it produces its own heat through the combustion of the fuel. If a car struggles to start initially, but restarts fine once the engine has warmed, this is another useful indicator of a likely fault in this system.
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Testing the Glow Plugs
There are many ways of testing glow plugs, the relay, and other components within the system. Today, I am going to show you one of the simplest ways of testing the glow plugs themselves.
The Multimeter (Test Meter)
A multimeter is a multifunctional piece of test equipment, ideal for electronic and auto-electrical testing applications. Here I am using the DMM301 Automotive Multimeter by Draper, but any basic multimeter should do the job.
Crocodile Clips (Optional)
Another useful tool are crocodile clips. More specifically, the kind that can plug directly into our meters prob wires. I have already upgraded my meter with probes that use banana plug terminals, allowing me to disconnect the standard meter probes from the wires and connect a range of alternative attachments. Using crocodile clips will allow us to clip our meter to the glow plug connections, without having to hold the probs by hand.
The Test
Electrical elements, such as the heater elements within the glow plugs, primarily consist of a coiled wire. When voltage is applied to this wire, its low resistance allows a large amount of current to flow through, raising the temperature of the coil and the surrounding heating tip. This coil of wire is calculated to produce the correct amount of heat, without drawing too much current.
To check the health of the glow plugs, we can read their resistance in turn. This may highlight a faulty glow plug, before any need to be removed. If all glow plugs show readings within an acceptable range, then the issue may be elsewhere.
What to Expect
The resistance of a glow plus heating element is usually very low, typically around 1 Ohm. Therefore, if we put our multimeter in resistance mode, signified by the omega symbol representing resistance (Ω), we should read a value very close to this on a working glow plug.
If the resistance is too high, this can indicate damage to the heating element. If there is damage to the element or the connections to the terminals, this will increase the circuit's resistance. This increase in resistance will hinder the amount of current that can pass through, reducing the effectiveness of the glow plug.
If the meter reads "OL", this signifies that the element or another internal connection has failed. The reading "OL" means "Over Load", suggesting the resistance is too high to read. This can occur if the element has broken, as the meter is effectively attempting to read an "infinite" resistance between the two test leads.
If the meter reads a value between 0.5Ω and 1.5Ω, then the glow plug itself is likely functional.
If the meter reads near 0Ω, then this suggests an almost direct continuity between the two terminals, and that the glow plug has most probably shorted out inside. If the windings have shorted to the outer casing, the current bypasses the rest of the heating coil, preventing it from heating up.
Performing the Test
Before performing the test, you will need to disconnect the cables going to all the glow plug anodes. You also need to ensure the meter is in resistance mode (Ω) and that the test leads are connected to the correct socket options on the meter.
Next, you need to connect one side of the meter to a glow plug's anode terminal, and the other side to the body of the engine block.
This test will read the resistance from the anode, through the glow plugs' internals, to the cathode body screwed into the engine.
Repeating this test for each glow plug in turn will reveal any outliers when comparing the results of each plug. If one or more plugs are reading values vastly different from the others, then this indicates that at least one plug may be at fault.
It is important to take readings from the ground of the engine block and not from the battery. Doing so may add additional resistance to the readings or mistakenly highlight a bad engine ground as a glow plug fault.
If no plug seems to be at fault, then other areas to test include:
Checking the fuse going to the glow plug module.
Checking the resistance through the engine ground link.
Checking the resistance between the glow plug wire and the glow plug module.
Verify that the glow plug module is outputting a voltage on each plug wire when the ignition is first turned on.
Checking the live and grounded supplies to the glow plug module.
Other, more thorough tests (Included in later tutorials)
Example Demonstration
In this demonstration, we will be performing this test on glowplugs that have already been removed, following a replacement. The lifespan of glow plugs fitted within the same engine usually follow very closely, so if one plug has gone faulty with age, then it is always recommended to change the others as a group.
A Working Glow Plug
The following image is a test of a working glow plug. We can see from the meter that the measured resistance value was within the 0.5 Ω to 1.5 Ω range.
A Faulty Glow Plug
The following example is a reading taken from a faulty glow plug. We can see that this resistance is measured in kilo ohms (kΩ), more than a thousand times the expected range. In some cases, glow plugs may have different acceptable ranges, very different from 0.5 Ω to 1.5 Ω. However, if one plug reads a much higher resistance than the others (such as in this case), this can indicate an outlier even when the acceptable range is unknown.
It is also visible that the colour of the faulty glow plug tip is different in comparison to the working plug. This can also be a good indicator of what is occurring within the cylinder, as with spark plugs from a petrol engine.
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This concludes a quick walkthrough of testing glow plugs with a multimeter. Thank you for reading, and feel free to contact us through social media with any suggestions for future tutorials.
At Rewired Customs and Classics, we take pride in ensuring that our information is as accurate as we can achieve. Feel free to take a look at some of our reference materials used to fact-check our post.
Bibliography
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‘Spark plugs vs Glow plugs | CHAMPION’, Ignition - Filters - Braking | Champion Parts. Accessed: Oct. 13, 2025. [Online]. Available: https://www.championautoparts.com/en-gb/news/spark-plugs-glow-plugs.html
[2]
‘What are glow plugs | Halfords UK’. Accessed: Oct. 13, 2025. [Online]. Available: https://www.halfords.com/motoring/car-parts/advice/what-are-glow-plugs.html
[3]
‘What are glow plugs? Replacement guide’. Accessed: Oct. 13, 2025. [Online]. Available: https://www.rac.co.uk/drive/advice/car-maintenance/what-are-glow-plugs-maintenance-and-replacement-guide/
