What are glow plugs? Maintenance, fault and replacement guide

Glow plugs are an essential part of a diesel engine. These small but essential items help start the engine in cold weather and faulty glow plugs can result in a breakdown.

Car maintenance is a vital part of owning a vehicle – and knowing when and how to replace your glow plugs is important.

An engine is made up of many complicated components that made it all work in harmony – so ensuring that your glow plugs are fitted correctly and in full working order is something that you should always be monitoring.

This guide looks at everything you will need to know about glow plugs and keeping your car in the safest condition possible.

How much does it cost to replace glow plugs?

The average cost to replace glow plugs is around £212. This includes parts and labour.

However, the price you will pay can vary due to the make, model, age and condition of your vehicle – as well as what type of glow plug you wish to have in your vehicle.

^{Data from whocanfixmycar, correct as on April 2025.}

Common signs of faulty or worn glow plugs

If your car's glow plugs are faulty or worn out, you will usually have one or more of the follow problems with your car:

• Engine won't start
• Engine takes longer than usual to start
• Rough idling or engine misfire
• Poor fuel economy
• Black or white smoke coming from the exhaust

What are glow plugs?

Glow plugs are electrical heating devices used in diesel engines to help start the vehicle.

They work by compressing air in the cylinder and then injecting diesel fuel into the engine.

These are the diesel equivalents to spark plugs for petrol engines.

One of the main differences is that diesel engines rely on the high temperature created by the compressed air to ignite the fuel.

However, in cold weather, the air temperature may not be high enough to ignite the diesel fuel effectively. This is where glow plugs are important for drivers of diesel cars.

They are used as heating tools that are installed in the combustion chamber of each cylinder of the engine. When the engine is started, they preheat the air so that the start-up is smooth and reliable.

After the engine has been running, and the temperature is regulated, the plugs turn themselves off.

Due to their importance for winter and colder weather – it is vital that they are working during these conditions.

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How do glow plugs work in diesel cars

Glow plugs are primarily used in diesel engines. Diesel fuel requires higher temperatures to ignite compared to petrol-powered vehicles.

This is why glow plugs are used to preheat the air in the cylinders to ensure that the engine can start and continue at the correct temperature.

In petrol engines, spark plugs are used for ignition instead.

Spark plugs create an electric spark to ignite the air-fuel mixture in the engine cylinders. This is why there is no need for glow plugs in petrol cars as they rely on spark plugs to initiate the combustion process.

How many glow plugs are in a car?

In most diesel-powered vehicles, there is typically one glow plug per cylinder. This would mean that in a six cylinder engine, there are six glow plugs.

However, this can vary depending on engine design, manufacturer, and quality of the engine.

It can also depend on the engine size and vehicle model or modification.

The information will be outlined in the owner’s manual.

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How to remove and replace glow plugs

Removing and replacing the glow plugs is a delicate process that requires extensive mechanical knowledge, protective clothing and specialist tools.

When a professional mechanic replaces a glow plug, they will typically follow these steps:

1. Open the bonnet, and locate the cylinders. Open them up and use a socket wrench to remove the old glow plugs
2. Carefully insert the new plugs and tighten them using the same tool
3. Make sure that the plugs are properly tightened, following the torque specifications for your vehicle (usually details in the car owner’s manual)
4. Reconnect any wires or connectors that were removed when they initially disconnected the plugs
5. Close up the cylinders and close the bonnet
6. Start the engine and see if the engine is running smoothly
7. If the engine has trouble starting or runs roughly, there may be a further issue with the engine that requires investigation.

Please note, glow plug replacement should only be carried out by a mobile mechanic or a qualified professional at a local garage. 

If you are incorrectly replacing or removing a glow plug, it could cause serious damage to your vehicle and may also result in an injury to yourself.

In this situation, take your vehicle to a trusted local garage or call a mobile mechanic.

Glow plug replacement

How long will glow plugs last?

Glow plugs normally last for around 80,000 to 100,000 miles - although this can depend on the brand of the glow plugs and build quality.

Driving style and vehicle maintenance can also impact glow plug lifespan.