The glow plug warning light informs diesel drivers that the glow plugs are heating up. (They need to get hot enough to start the engine quickly and efficiently.)
However, if the glow plug light stays illuminated, this could be a sign that there’s something wrong with your engine.
In this guide, we’ll list the common causes of a glow plug warning light. We’ll also cover the risks around driving with this light on – and the action you should take when it appears. Finally, we’ll share expert tips to help you avoid glow plug issues.
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Why is my glow plug warning light on?
The glow plug warning light can be found on the dashboard of most diesel vehicles, along with the other dashboard warning lights.
The symbol resembles a coiled or looped wire. It should light up temporarily when you switch on the ignition, indicating that the glow plugs are heating up.
You should wait until the glow plug light switches off before starting the engine, as the glow plugs need to complete their heating cycle to start the engine efficiently.
What does the glow plug warning light mean?
While the glow plug warning light should only appear temporarily when you start your ignition, certain issues may cause it to stay lit, including:
- Cold weather: The glow plug light may appear for slightly longer when you switch on your ignition during the winter, as the plugs may need extra time to heat up.
- Worn-out glow plugs: If your glow plug light comes on without ignition, the plugs may be worn out.
- Failing glow plug: If the glow plug light stays lit for over 15 seconds, your glow plugs may be failing, meaning they will need to be replaced.
- Malfunctioning sensor/timer: Your glow plugs will not work properly if the sensor-equipped timer has malfunctioned.
What does it mean when the glow plug warning light is flashing?
A flashing glow plug warning light indicates that there are issues with your glow plugs, the timer, or the diesel engine itself.
If you leave this light unchecked, you could develop serious engine issues. Therefore, if you see the glow plug warning light flashing, take your car to a mechanic as soon as possible.
Is it safe to drive when my glow plug warning light is on?
We would strongly advise against driving with a glow plug warning light, as there could be an issue with your glow plugs, their sensor, or even the engine.
By driving with a glow plug warning light, you run the risk of developing serious engine problems. The engine may struggle to start again, or even shut off whilst you’re driving – which could be extremely dangerous.
If the glow plug light appears while you’re driving, pull over as soon as it is safe, then call your breakdown provider to take you to the nearest garage.
Will my car fail its MOT if the glow plug warning light is on?
If your glow plug warning light switches on for longer than 15 seconds, but disappears shortly afterwards, you may receive an advisory rather than fail your MOT.
However, if your glow plug warning light flashes, or stays on constantly, your car will fail its MOT, because leaving this light unchecked could cause serious damage – and potentially, put you and other road users at risk.
How does the glow plug warning light work?
- The glow plugs are connected to sensors, which monitor the temperature of the engine, and signal for the glow plugs to switch on as needed.
- The glow plug warning light appears when the car’s ignition is turned on to indicate that the glow plugs are heating up to start the engine more efficiently.
- As the glow plug warning light is connected to the glow plug and sensor system, it may illuminate when there are issues with the glow plugs, sensors, or the engine.
What to do when your glow plug warning light comes on
If your glow plug warning light appears, it’s important to get help right away. Here are the steps to follow:
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Do not drive the vehicle. A glow plug warning light may indicate deeper issues with the glow plugs, sensors, or even the engine itself.
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Switch on your ignition and note down what happens with your glow plug warning light. Is the light flashing? Does the light switch on as expected? How long does the light stay on after you start the car?
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Call your local mechanic and explain the problem. They will advise you on whether it is safe to drive your vehicle to the appointment. If not, ask to be towed to the garage, or call your breakdown provider to take you there.
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Following your appointment, you’ll be briefed on the issue causing the warning light – and how it can be resolved.
Tips for preventing glow plug issues
Glow plugs typically last between 80,000 and 100,000 miles. Of course, the exact lifespan of your glow plugs will depend on factors such as their make and quality, your driving style – and how well you take care of them.
Here are a few expert tips to help you avoid glow plug issues:
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Regularly check and clean your glow plugs. Keep an eye out for cracks, wear, or a build-up of carbon deposits.
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Monitor how long it takes for your glow plug warning light to switch off. If you notice it’s taking longer than expected, take your car to a mechanic.
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Replace your glow plugs when they’re nearing the end of their lifespan, whether that’s because they’re approaching the 80,000-mile mark, or there is already noticeable wear and tear.
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