Diesel Glow Plugs: What They Do and Signs They Need Replacing

What are the signs of a faulty glow plug? The most common signs of a faulty glow plug are hard starting on a cold morning, misfiring or rough running until the engine has warmed, and a glow plug warning light that flashes or stays on after warm-up. You may also notice white or black exhaust smoke and higher fuel use. A faulty glow plug can stop the DPF cleaning itself, so have it checked promptly.

What are diesel glow plugs and what do they do?

Modern diesel engines tend not to require pre-warming in normal weather, unlike older diesel engines. Yet, an electrical heater/glow plug is still fitted. Therefore, you might not notice that a glow plug has deteriorated, or stopped working, until your car appears hesitant to start on a cold morning. It may even misfire, or run unevenly until it has warmed. 

Signs of a faulty glow plug

A faulty glow plug can show itself in several ways:

• Hard starting when the engine is cold, and in winter the car may not start at all
• Misfiring or rough running until the engine has warmed
• White smoke on start-up or black smoke when driving
• Higher fuel consumption
• A glow plug warning light that flashes, or stays on after the engine has warmed up. The light staying on for a few seconds when you switch on the ignition is normal, as the plugs are heating. Our guide explains what your dashboard warning lights mean.
• A blocked DPF, because the filter cannot complete DPF regeneration

Other faults can cause the same symptoms, so have a garage confirm the diagnosis.

How can faulty glow plugs affect your DPF?

On the latest diesel engines, glow plugs have another role to play. Should one of them stop working, it can prevent the diesel particulate filter (DPF) from self-cleaning. During DPF regeneration, the engine control unit uses the glow plugs to help raise the exhaust temperature. Therefore, a problem with a blocked DPF is not down to the filter but the glow plugs. Unless the underlying glow plug problem is addressed, paying a garage to clean the DPF can be pointless.

How long do glow plugs last?

Unlike spark plugs, glow plugs generally have no manufacturer-specified replacement interval, so glow plug replacement is usually prompted by symptoms rather than a service schedule. How long they last depends on the number of heat cycles, meaning engine starts and DPF regenerations, more than on mileage. Frequent cold starts and short journeys that never bring the engine to temperature wear them out sooner. Check your handbook for your manufacturer’s guidance, and ask for the glow plugs to be checked at a service. GEM explains what happens when you skip a car service.

Is it difficult to replace glow plugs?

Whether paying a garage to do the work, or you conduct the work at home, consider that replacing glow plugs is not always straightforward. They are delicate components and can snap, as they are extracted from the cylinder-head. Removing a broken glow plug may necessitate involved engine dismantling and put the car out of action for some time.

Should you replace all glow plugs at once?

Glow plugs should be replaced in engine sets. Changing just one is not best practice. In some cases, the control relay should also be renewed, adding to the expense. So, if a garage recommends these extra parts, you are not being hoodwinked and the recommendation is correct. 

Glow plug FAQs

Can you drive with a faulty glow plug?

For a short time, usually, but it is not advisable. A faulty glow plug will not normally stop the engine at once, but expect hard cold starts, rough running, weaker performance and higher emissions. It can also stop DPF regeneration and lead to a blocked filter, so get it checked as soon as you see a flashing or constantly lit glow plug warning light. Avoid driving if the engine will not start, runs badly or misfires, or shows heavy white smoke or other warning lights.

Can a faulty glow plug cause an MOT failure?

Not directly, as the MOT does not include a check on the glow plugs themselves. The problems a faulty glow plug causes can fail the test, though. An illuminated engine management light is a failure, as is any visible smoke from a diesel car fitted with a DPF, and a car that exceeds the diesel smoke limit also fails. Evidence that a DPF has been tampered with is a failure too, and our guide covers diesel particulate filter removal.

What is the difference between glow plugs and spark plugs?

Spark plugs ignite the fuel and air mixture in a petrol engine with an electric spark. A diesel engine ignites its fuel through compression, so glow plugs do not ignite anything. They are heating elements that warm the combustion chamber to help the engine start from cold, and on newer diesels they also support DPF regeneration. Spark plugs usually have a service interval, but glow plugs generally do not.

Can you test glow plugs yourself?

Yes, with a multimeter, if you are confident working on a car’s electrics. Check the battery voltage first, as glow plugs only work correctly with the right voltage. Then disconnect and clean the connectors, measure the resistance of each plug and subtract the meter’s own resistance. Compare the readings with the range in your repair manual.

Every plug should read close to the others, and one that is out of range points to a faulty glow plug. If it fails, glow plug replacement is needed, and the whole set should be replaced together. If you are not sure, ask a garage, as a plug that snaps on removal is a bigger job.

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