How They Work

Glow plugs are heating devices that are used in diesel engines to preheat the air in the combustion chamber before the engine is started. They work by converting electrical energy into heat energy, which is used to warm the air in the cylinder.


When the engine is turned off, the temperature inside the cylinder drops, which can make it difficult for the diesel fuel to ignite. The glow plugs are used to preheat the air in the cylinder to a temperature that is high enough for the diesel fuel to ignite. This helps to ensure that the engine starts easily, even in cold temperatures.


When the key is turned to the "on" position, power is sent to the glow plugs, which heat up. The heating process typically takes about 5 seconds. Once the glow plugs reach a certain temperature, the engine control module sends a signal to the fuel injectors to start spraying fuel into the cylinders. The fuel then vaporizes and burns, starting the engine.


Improving cold starts is just one of the functions of modern glow plugs. They also contribute towards the reduction of engine emissions by generating a ‘post-glow’ for several minutes after ignition, which helps to ensure the diesel fuel completely combusts during the warm-up phase. Glow plugs also benefit the health of the exhaust gas recirculation (EGR) valve by producing what is known as an ‘intermediate glow’. This helps to prevent the EGR valve from carbonising, which affects its ability to reduce the vehicle’s particle and nitrogen oxide emissions.


Glow plug technology has evolved over time, particularly in terms of the materials used.


In the past, copper was one of the most common materials used in glow plugs. Copper is a good conductor of heat, but it has a tendency to wear out quickly and can cause the glow plug to fail. This led to the development of more durable materials like nickel-chromium and nickel-copper alloys. These materials have a higher melting point and are more resistant to wear and tear, which makes them a good choice for high-performance engines. This improved the lifespan of the glow plugs and their reliability.


Recently, ceramic material has been used in glow plugs, they have a high melting point, good thermal conductivity and they are resistant to wear and tear. They also have a smart technology that allows them to communicate with the engine control module and provide information on the glow plug's performance, temperature and voltage. These "Ceramic glow plugs" or "Self-regulating Glow plugs" have a longer lifespan and reduce emissions, improve fuel economy and increase the life of the engine.

Why They Fail

The lifespan of a glow plug can vary depending on several factors such as the quality of the glow plug, the engine's usage, and maintenance. However, some glow plugs may need to be replaced before that if they fail or show signs of wear. The good news though is that  not only are glow plugs incredibly long-lasting – their usual lifespan is around 150,000 kilometers - they’re also amongst the most cost-effective car parts to replace.


Overheating: Glow plugs are designed to heat up to high temperatures, but if they overheat, they can become damaged. This can happen if the engine's cooling system is not working properly, or if the glow plugs are not sized correctly for the engine.


Voltage spike: Glow plugs are powered by the battery voltage, if there is a sudden spike in the voltage, it can cause the glow plugs to fail.


Age: Glow plugs have a lifespan, and over time, they will wear out and fail. Even with proper maintenance, it's expected that they will eventually need to be replaced.


Collateral Damage: If other parts in the vicinity of the glow plug fail, they can in turn damage the glow plug. Examples of this include: If the pistons are damaged, if the valves are broken, if there is a seized piston ring, and if the injectors have dripped on the glow plugs and eroded them


Poor quality: Using low-quality or incompatible glow plugs can cause them to fail prematurely.

It is important to note that, a failing glow plug can cause the engine to be harder to start or not start at all, and can also result in poor fuel economy and increased emissions. Regular maintenance, such as checking and replacing the glow plugs as needed, can help to prevent or minimize the likelihood of glow plug failure.



Signs That Your Glow Plugs Need Replacing

Here are some signs that a glow plug may need replacing:

Hard starting: If the engine is difficult to start, especially in cold temperatures, it could be a sign that one or more of the glow plugs are failing.


Misfiring: A failing glow plug can cause the engine to misfire, which can result in poor performance and increased emissions.


Warning light: Some vehicles have a warning light on the dashboard that will illuminate if there is a problem with the glow plugs.


Poor fuel economy: A failing glow plug can cause the engine to run less efficiently, which can result in poor fuel economy.


Excessive smoke: A failing glow plug can cause the engine to produce excessive smoke, which can be a sign of poor combustion.


Poor idling: A failing glow plug can cause the engine to idle poorly, which can result in vibrations and rough running.

It is important to note that, these symptoms can also be caused by other issues, such as a failing injector or a problem with the engine's compression. A qualified mechanic can diagnose the problem and determine if the glow plugs need to be replaced.



Tips To Prevent Premature Glow Plug Failure

Regular inspections: Having regular inspections of your vehicle's glow plugs is important to identify any issues before they become serious. This should be done at least once a year, or more often if you notice any symptoms of wear or damage.


Proper maintenance: Regularly maintain your vehicle by changing oil and air filters, checking the battery voltage and getting regular services as per the manufacturer's recommendations. This can help to ensure the proper operation of the engine and prevent problems with the glow plugs.


Avoid overheating: Avoid overheating the engine by ensuring that the cooling system is in good working order and the vehicle is not being driven in extreme temperatures. Overheating can cause the glow plugs to fail prematurely.


Use good quality glow plugs: Use high-quality glow plugs that are designed for your specific engine, as using lower quality or incompatible plugs can shorten their lifespan.


Proper Voltage: Make sure the battery voltage is maintained at the recommended level, as low voltage can cause the glow plugs to fail.


By following these tips, you can help prevent glow plugs from failing prematurely and ensure that your diesel engine is running efficiently and reliably. Regular maintenance and inspections will help to extend the lifespan of your glow plugs and prevent costly repairs.



How We Can Help

At StartMyCar, we understand the importance of high-quality and reliable glow plugs for diesel engines. That's why we stock the full range of NGK glow plugs, which are widely regarded as being superior in terms of quality and performance.


NGK glow plugs are made of durable materials such as nickel-chromium and nickel-copper alloys, which have a longer lifespan and better resistance to corrosion. They are also precision-engineered to ensure a perfect fit and maximum performance, and are designed to improve the performance of diesel engines by providing faster heating times, better combustion and improved fuel economy.


By choosing NGK glow plugs from StartMyCar, you can trust that you are getting the best quality and performance for your vehicle. Whether you're a professional mechanic or a DIY enthusiast, you can trust that our team of experts are here to help you find the right glow plugs for your vehicle.


If you have any technical questions related to glow plugs and how to replace them, we invite you to reach out to us at: