Why might the glow plug warning light in a diesel car come on?

• What do glow plugs do?
• Do glow plugs need replacing?
• Don’t ignore the warning light

Although the glow plug warning light is designed to come on for a short period before the engine is started, it shouldn’t be ignored if it lights up while the engine is running, but why?

As stated, the glow plug warning light will come on as the vehicle’s ignition is turned on, because this warning is to inform you that the glow plugs are going through their heating cycle in order for them to be hot enough to make starting the engine quick and efficient. This is why you should wait until the light goes out, before starting the engine.

 

What do glow plugs do?

Glow plugs play a key part in minimising the environmental impact of diesel emissions, alongside ensuring engine reliability and performance. To start combustion within a diesel engine, air is compressed in a cylinder to raise its temperature and pressure, fuel is then added to start the combustion cycle to allow the engine to run. If the temperature of the air is too low, then some of the fuel that is supplied may not be burnt. Unburnt fuel then turns into carbon and is released via the exhaust, causing environmental damage. To minimise the amount of unburnt fuel, glow plugs provide extra heat to the air and ensure that the cylinders reach the required temperature for a clean combustion cycle, as well as to help the engine start more easily. 

The engine management system can also trigger glow plugs to provide heat to the diesel particulate filter (DPF), which burns toxic soot particles before they leave the exhaust in a process called DPF regeneration. This burns the soot particles, meaning it won’t have an adverse effect on the environment. 

 

Do glow plugs need replacing?

Like all engine components, glow plugs are wearing parts and will at some stage need to be replaced, which is why vehicle manufacturers specify changing the glow plugs at specific intervals as part of the vehicle’s service schedule.

 

Don’t ignore the warning light

Despite this requirement, troubles can still occasionally occur, which is why the warning light must not be ignored if it fails to go out before the engine is started, or comes on during its normal running. Generally, the most likely reasons for the warning light are an underlying issue with one/all of the glow plugs or the sensors that control them, but it could also indicate a problem with the engine itself.

If the warning light is staying on for longer than 10 to 15 seconds, before the engine is started, it’s likely that the glow plug is losing its power efficiency and it will need to be replaced. However, if it stays on for even longer or starts to flash, it’s probable the glow plug has suffered a complete loss of power, so must be replaced immediately.

When and for how long the glow plug is heated is governed by sensors and if these malfunction, it will have a direct effect on the engine and its efficiency. In the worst case, it is possible that the engine or DPF will suffer damage, so immediate remedial action by a professional technician will be needed to find the underlying cause and put it right.