How to test and replace a glow plug

Do you have a diesel car? Then you might be familiar with glow plugs, which are used to create ignition in diesel vehicles. They are the equivalent of spark plugs in a petrol car but work slightly differently to start the engine.

Glow plugs generally have a long life span but as with any part, will deteriorate over time with normal wear and tear. Today we’ve outlined a basic guide on  how to test and replace your glow plugs.

Before continuing, please make sure you read all parts of this article before attempting any repairs. We recommend having a level of familiarity with engine mechanical and electrical systems and the correct tools to proceed. If you don’t feel confident with this, just nip into your nearest Natrad store for specialist advice!

If you do, the first step in the process is to identify whether your glow plugs need replacing. To find out the most common glow plug failure symptoms, read on.

Natrad stocks a range of high-quality glow plugs to suit a variety of vehicle applications. Read about our range here!

How to test and replace a glow plug

Before we dive into failure symptoms, it may help to understand what a glow plug does. Glow plugs assist ignition by heating up the air and fuel (diesel) mixture. Reaching a high enough temperature allows it to combust under pressure. This can be at over 200° C.

Glow plugs generally heat in three stages. The first is pre-heating, where they heat up rapidly in order to begin combustion. The second is maintaining temperature to ensure optimal combustion. And finally, post-heating where glow plugs will remain heated for a short period after the engine has started.

Failed glow plugs — what are the signs?


Glow Plug


If you’ve noticed something odd with your engine lately, it could be your glow plugs. Some of the most common failure symptoms include:

  1. Check engine light: the check engine light may appear for several reasons, but it could indicate an issue with your glow plugs.
  2. Hard starting: this refers to a difficult to start car when you turn on the ignition. This could be due to damaged or old plugs, which results in too low a temperature to start the engine with.
  3. Misfiring: also due to a lack of high temperature, misfiring happens when the ignition of the fuel to air mixture doesn’t burn properly. This feels like hesitation or shaking when the car is running.
  4. Rough idling: when the combustion chamber can’t reach the right temperature, it may also result in rough idling once the car has started.
  5. White smoke: smoke is common when there is not enough heat to burn the fuel. This can happen on colder mornings before the car has fully warmed up, but faulty glow plugs can also cause smoke on startup.

If any of these sound familiar, your glow plugs could be the culprit. To test them yourself, refer to our guide below. If you’re not comfortable DIY-ing, talk to one of our technicians today!

For the best quality fitment, talk to one of our specialists at Natrad. Our range is suited to a huge variety of vehicles. 

Step-by-step guide

We have created a basic guide to testing and replacing your own glow plugs. You can find our replacement guide further down!

First off, what will you need?

  • Basic toolset to access and remove glow plugs (includes a socket and rachet set)
  • Service manual/owner’s manual
  • Digital multimeter
  • Flashlight (optional)

If you need to replace your glow plugs after testing, head to your nearest Natrad store where our qualified technicians can help!

Testing your glow plugs

To test the function of your glow plug, you’ll need to start with your multimeter. This digital device measures electrical circuits.

Using your multimeter.

Ohms is the unit of measurement necessary for this test. Ohms measures resistance and is represented by the omega symbol (Ω). You will be using Ohms to determine if the glow plug meets the acceptable resistance range in your vehicle.

1. Check your owner’s manual

To find the acceptable resistance range of your glow plugs, you can look in your owner’s manual.

3. Check your multimeter is working

Make sure you multimeter is working properly by touching the tips of the metal pins together. The reading should be as close to 0 as possible.

However, if you get a reading of 1.0 or more, this means the multimeter isn’t working properly or that the resistance range is too high. Try setting it to the lowest setting and try again.

4. Test your car battery

To rule out issues with the car battery, you should also test it using a multimeter. Glow plugs only work correctly when they are supplied with the right voltage. So, set your multimeter to DC voltage. You should only need to measure about 13 Volts.

Connect your negative (black) lead to the negative car battery terminal and your positive (red) lead to the positive battery terminal. You should get about 12.5 Volts. If all is okay, then you can proceed with testing the glow plug.

5. Locate and clean your glow plugs

Glow plugs may be easy to reach depending on your vehicle configuration. They usually have a cap or connector covering them, so once located, remove this first. Look at the surrounds of the glow plug. If there is any rust, corrosion or loose-fittings (cover) then cleaning this might improve their performance.

6. Test your glow plugs

To do this, make sure your multimeter is set back to Ohms and place your leads as follows:

  • Touch the negative lead (black) to a ground point. You can consult your owner’s manual to find where this is – but it is usually a bolt connected to wires either from the car battery (negative terminal) or alternator.
  • Touch the positive lead (red) to the top of a glow plug. You should now be getting a reading.

If all of the glow plugs read within the acceptable range determined by the owner’s manual, there is another reason for issues with your car. However, if one or more reads abnormally high, this may be affecting engine performance. It might be time to replace one or more of the plugs.

Replacing your glow plugs

replacing glow plugs in a diesel engine

If you have found issues with your glow plugs, now we get to the nitty-gritty. How to replace your glow plugs!

First off, you will need:

  • Manual torque wrench/socket wrench
  • Replacement glow plugs
  • Pliers
  • Engine cover (if you wish to replace your existing one)

Let’s get started on replacing your glow plugs.

1. Disconnect the battery

Working with an electrical system means safety should be paramount. Disconnecting the battery reduces the chances of getting caught in the current.

3. Disconnect the connector

Remove the connecter or cap fastened to the glow plug. This is where the pliers might come in handy, but be careful to not break the wiring.

4. Remove glow plug

To remove the glow plug(s), you’ll need a socket wrench (also known as a socket and ratchet). You’ll need to place the ratchet over the plug and turn left (counterclockwise). You might require an attachment known as a deep socket. It’s advised you rotate very slowly to avoid breaking the plugs.

5. Install new plug

When placing in the new plug, make sure the area is as clean as possible. Then, screw the plugs in by hand and tighten using a torque wrench. It is crucial to not over tighten, so make sure to check the correct torque settings for your car in the owner’s manual.

6. Re-attach connectors

Once all the plugs are in place, make sure to re-attach connecters. (This does not mean reconnecting the battery just yet). Rinse and repeat for each plug.

7. Place engine cover

If your engine cover was removed during the process of testing or replacing your glow plugs, now is the time to put it back in place. If it was damaged at all during the process, it is a good idea to replace it as soon as possible.

8. Reconnect the battery cable

You’re finished! Now that everything is in place, you can reconnect your battery.

What now?

If you got stuck or didn’t feel comfortable taking on testing or replacing glow plugs, get in contact with your nearest Natrad store.  There’s a nationwide network of workshops that are available to help, with plenty of replacement glow plug options.

Natrad has a range of high-quality glow plugs for diesel engines available from your nearest store today.