What Are Glow Plugs & When To Replace Them

In this article, we will explain everything you need to know about glow plugs and when a change may be required. Use this ultimate guide for troubleshooting glow plug problems and finding the best solution for a replacement. If you have good maintenance skills, follow our in depth guide on how to extract and replace worn glow plugs. 

Bosch Glow Plug
Photo Credit: Bosch

What is a Glow Plug?

A glow plug is a cylindrical component containing a heating element at one end and an electrical connection at the other. The body consists of a central electrode and regulation coil (the reason behind the coil-shaped amber warning light). Below the connectors is a hexagonal nut used for tightening and unscrewing them into place.

When an electrical current is passed through the tip it will emit visible light similar to that of a hair dryer. This warmth is used to significantly heat diesel before ignition making the vehicle easier to start and aid with combustion when the engine block is cold. The glow plug filaments themselves are made of durable rare metals such as iridium or platinum.

Is a glow plug a spark plug?

Despite the similar sounding name, a glow plugs and spark plugs work in very different ways. Contrary to its diesel warming counterpart, the spark plug is used to directly ignite the fuel/air mixture. It will use electricity from the battery to create an electrical charge in the combustion cylinder. Petroleum is much easier to combust than its oily diesel cousin. The lifespan of a spark plug is generally much shorter due to constant use and contamination from combustion.

How does a glow plug work?

Just how does a glow plug function exactly? As you turn the key, your glow plugs will heat up significantly to over 800 degrees, that’s hot enough to emit visible light. Ceramic tipped glow plugs can get to a whopping 1300 degrees. Diesel and compressed air will then be injected into the combustion chamber (or pre-chamber depending on the engine type) where the glow plug tips will effectively heat the fuel. This mixture will ignite under pressure and the force created will drive the pistons.

Types of Glow Plugs

Older vehicles used the “thermostart” variety which was considerably less effective than the modern “in cylinder” variety. If you have ever driven an older diesel car you may remember “glowing”. This is where you would turn the key partially and wait for the glow plugs to preheat which was indicated by waiting for the amber glow plug indicator light to disappear.

The modern “in cylinder” type will have a glow plug for each cylinder making the process quicker and more effective. This will either be in the combustion chamber for direct injection engines or in the pre-chamber for pre-chambered engines. Once the cylinder is up to temperature (usually within a minute or so) any subsequent starts will be quicker and more efficient.

Symptoms of Bad Glow Plugs

Engine Maintenance
Image Credit: Unsplash

Difficulty starting

Taking a few extra turns or difficulty starting will be the most common symptom of worn glow plugs. This difficulty will become more prevalent as the air temperature drops during the winter months. Once the engine is warm, any attempts to restart the engine may happen far quicker. Push-button start diesel vehicles can have a small delay between pressing and ignition, this is by design and a normal function.

Glow plugs warning light

Some cars will have an inbuilt sensor to tell the driver when the glow plugs are no longer functioning correctly. This will be in the form of a flashing yellow light in the shape of a coil. Other cars may have a warning message on the digital display.


Plumes of exhaust smoke are a tell-tale sign of glow plug issues, in particular shortly after ignition. This is because the fuel isn’t hot enough to combust properly, however smoke can be a sign of other engine problems. Take a look at our article on Diesel Exhaust Smoke if you are having prolonged issues with this.


As the engine is trying to combust colder diesel & you may notice juddering under acceleration with a decrease in power.

DPF Problems

You may notice a DPF warning light appearing. A DPF regeneration will halted when glow plug are not functioning correctly. Glow plugs are required to pulse with significant heat during a regeneration. Prolonged glow plug problems can cause serious damage to this expensive component. Anything DPF related is worth diagnosing straight away.

Decreased MPG

Like a variety of diesel engine components, if the glow plugs are not functioning properly, you can expect a drop in MPG. With the price of diesel these days every mile per gallon counts.

When should I replace my glow plugs?

Glow plugs are incredibly hardy components, with the expected lifespan being 90,000 miles. Lower quality brands or frequent cold weather starts may impact the durability greatly. Check the vehicle handbook to see when the manufacturers recommended intervals are. Some service schedules will include glow plugs as part of routine maintenance when the vehicle reaches a certain age or mileage.  Likewise, if you get any of the symptoms above it is worth troubleshooting or taking into a garage for diagnostics.

How To Change The Glow Plugs

Photo Credit: Unsplash

With a basic grasp of vehicle maintenance, you can change worn glow plugs yourself. However, it’s best left to an experienced mechanic. Over time the heat and pressure can wear or fuse the thread making them difficult to remove. If not undertaken carefully, the glow plug can snap leaving an exceptionally time consuming and costly job.

Competent with vehicle maintenance? Follow our guide for changing your glow plugs:

  • Firstly, locate the glow plugs and find how many you need to change, as a rule of thumb there is one for each cylinder but there could be fewer dependent on the model and engine type.
  • Find the correct glow plug for the engine. You don’t have to replace them with OEM spec components, there will be more cost-effective options open to you. However, using a respected brand such as Bosch is generally good practice. A higher quality replacement can reduce knocking, warm up faster and have better durability.
  • Soak the thread with WD40 or similar penetrating lubricant for at least a few days. This will slowly soak deep down into the thread making extraction easier and reducing the chances of snapping.
  • Remove the electrical connectors
  • With an extended torque wrench CAREFULLY apply pressure and slowly unscrew the glow plugs.
  • Double-check the glow plug specifications are correct for the engine. (they can cause damage if not a compatible size or voltage)
  • Add a little copper grease to the thread of the glow plug. This will prevent water from seeping through in the years to come.
  • Screw-in partly by hand if reachable to prevent cross-threading.
  • Set your torque wrench to the required setting and screw it into place. The correct torque settings are usually 8 – 10 NM.
  • Reconnect the electrical connectors.
  • Fire it up, enjoy efficient starting and improved MPG.


Whether you are having glow plug trouble or experiencing similar symptoms, it is always worth troubleshooting or taking into a garage. Getting annoying glow plug issues fixed early can prevent bigger problems in the future particularly with your DPF.

We hope that you have found this guide informative. If you have experienced your own glow plug problems please comment below or share via social media.

Andy Marchant
Andy Marchanthttps://autozilla.co.uk/
Andy has had a passion for cars from a young age. Since graduating from University where he studied journalism. Andy has been able to combine his love for literature and the automotive industry as a freelance writer.

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