Glow Plug Replacement Guide for Diesel Cars: Symptoms, Coil Lights & UK Costs
There’s nothing more frustrating than a cold UK morning when you turn the key and your diesel engine struggles to start. Long cranking, rough idling, clouds of smoke, or that familiar coil light glowing on the dashboard are often the first warning signs of failing glow plugs.
Glow plugs play a crucial role in diesel engines, especially in colder climates. When they start to fail, the effects go far beyond hard starting – fuel economy, emissions, and long-term engine health can all suffer.
In this comprehensive UK-focused guide, we’ll explain exactly how glow plugs work, the most common symptoms of failure, what the coil light really means, when replacement is necessary, and how much glow plug replacement typically costs in the UK.
What Are Glow Plugs?
Glow plugs are small heating elements fitted into each cylinder of a diesel engine. Unlike petrol engines, which rely on spark plugs to ignite the air-fuel mixture, diesel engines depend on compression. In cold conditions, compression alone isn’t always enough to achieve reliable ignition.
Glow plugs solve this problem by preheating the combustion chamber before and during startup. This allows diesel fuel to ignite cleanly and efficiently, even in low temperatures.
Modern glow plugs also play an important role beyond cold starting. Many engines use post-heating (after the engine has started) to stabilise combustion, reduce smoke, and improve emissions – a key factor for MOT compliance.
Definition
A glow plug is an electrically powered heating device used in diesel engines to raise combustion chamber temperatures during cold starts. By heating the air inside the cylinder, glow plugs ensure reliable ignition and smoother engine operation.
Types of Glow Plugs
Not all glow plugs are the same. The type used depends on engine design and emissions requirements:
- Metal (Pencil) Glow Plugs – Common in older diesel engines, reliable and cost-effective.
- Ceramic Glow Plugs – Heat up faster and withstand higher temperatures, often used in Euro 5 and Euro 6 engines.
- Pressure Sensor Glow Plugs – Advanced plugs that also measure cylinder pressure for precise engine management.
Using the correct glow plug type is essential. Fitting the wrong specification can cause starting issues or ECU fault codes.
How Glow Plugs Work
When you switch the ignition on, the glow plug control unit sends electrical current to each glow plug. Within seconds, the tip heats up to extremely high temperatures, warming the air inside the combustion chamber.
This heat allows diesel fuel to ignite quickly and evenly when the engine cranks. In many modern vehicles, glow plugs continue operating briefly after startup to stabilise combustion, reduce engine noise, and limit exhaust emissions.
Do Diesel Cars Have Spark Plugs?
No – diesel engines do not use spark plugs.
Petrol engines rely on spark plugs to ignite the fuel-air mixture using an electrical spark. Diesel engines, on the other hand, rely on compression ignition, where fuel ignites due to high pressure and temperature.
Glow plugs assist this process only during cold starts and warm-up phases. This difference is why glow plugs typically last longer than spark plugs but can be more difficult to remove due to carbon build-up.
Symptoms of Faulty Glow Plugs
Failing glow plugs often show warning signs long before complete failure. Identifying these early can help you avoid breakdowns and costly repairs.
- Hard starting – Extended cranking or failure to start, especially on cold mornings.
- Rough idle after startup – Engine vibration or uneven running for the first few minutes.
- White or black smoke – Unburnt fuel exiting the exhaust due to incomplete combustion.
- Poor fuel economy – Increased fuel consumption as the engine struggles to run efficiently.
If one glow plug has failed, the others are often not far behind. This is why many garages recommend replacing glow plugs as a complete set.
What Does the Coil Light on the Dashboard Mean?
The glow plug coil light (often resembling a coiled wire or pig’s tail) is the main indicator of glow plug system operation.
Under normal conditions, the light illuminates briefly when you turn the ignition on and switches off once the glow plugs are ready. You should wait for this light to go out before starting the engine.
If the light stays on or begins flashing, a fault has been detected and stored in the vehicle’s ECU.
Pre-heating
A solid coil light during ignition indicates the pre-heating phase. This is normal and essential in cold weather. The colder the temperature, the longer the pre-heating phase may last.
Flashing Coil Light (VW / Audi)
On Volkswagen, Audi, Skoda, and SEAT vehicles, a flashing coil light doesn’t always point directly to glow plugs.
Common causes include:
- Failed glow plug or glow plug relay
- Brake light switch fault
- EGR valve or turbo actuator issues
If the vehicle enters limp mode or loses power, it’s advisable to stop driving and have the fault diagnosed to avoid further damage.
Is It Safe to Drive?
Driving with a flashing coil light is not recommended. While the car may still run, continued driving can lead to poor combustion, increased emissions, and potential DPF blockage – an expensive repair.
When Should Glow Plugs Be Replaced?
Glow plugs do not have a fixed service interval, but they typically last between 60,000 and 100,000 miles. Cold climates, frequent short journeys, and stop-start driving can shorten their lifespan.
Replacement is recommended if starting issues appear, fault codes are present, or the vehicle fails emissions-related checks.
Mileage
As a general guideline, glow plugs approaching 80,000–100,000 miles should be inspected, even if no symptoms are present.
How Many Glow Plugs Does My Car Have?
Most diesel engines use one glow plug per cylinder:
- 4-cylinder engines: 4 glow plugs
- 6-cylinder engines: 6 glow plugs
- 8-cylinder engines: 8 glow plugs
Glow Plug Control Module and Relay Problems
Sometimes glow plugs are not the issue. A faulty glow plug control module or relay can prevent power from reaching perfectly good plugs.
Symptoms can closely mimic glow plug failure, which is why proper diagnostics are essential before replacing parts.
Glow Plug Replacement Process
Glow plug replacement is generally straightforward but carries risks if done incorrectly.
DIY vs Professional
DIY replacement can reduce costs significantly but requires mechanical confidence. Professional replacement offers peace of mind and reduces the risk of snapped glow plugs.
Risks of Snapped Glow Plugs
Glow plugs can seize in the cylinder head over time. Removing them incorrectly can cause the plug to snap, turning a simple job into a costly repair.
Extractor (Brief)
Specialist extractor tools exist to remove broken glow plugs, but success is not guaranteed. This is why many owners choose professional replacement.
Glow Plug Replacement Costs in the UK
Glow plug replacement costs in the UK vary depending on engine design, accessibility, and labour rates.
Average Cost
Most UK drivers can expect to pay between £100 and £250 for glow plug replacement.
Labour vs Parts
- Glow plugs: £10–£20 per plug (premium brands may cost more)
- Labour: £60–£100 depending on access and engine layout
Cost of Replacing 4 Glow Plugs in the UK
Replacing four glow plugs typically costs between £100 and £200, depending on whether the work is done at an independent garage or main dealer.
Frequently Asked Questions
Can you drive with a faulty glow plug?
Yes, but it’s not recommended. Continued driving can worsen emissions issues and cause DPF problems.
Should all glow plugs be replaced at once?
Yes. Replacing the full set saves labour costs and ensures balanced engine operation.
Do glow plugs affect MOT?
Indirectly, yes. Faulty glow plugs can cause excessive smoke and emissions-related MOT failures.
Conclusion + Call to Action
Glow plugs may be small components, but their impact on diesel engine reliability is significant. From cold starting to emissions control, maintaining a healthy glow plug system is essential.
If you’re experiencing starting issues or warning lights, don’t wait for a breakdown. Whether you’re replacing one plug or a full set, choosing the correct specification is key.
Not sure which glow plugs fit your vehicle?
Enter your registration number to find guaranteed compatible glow plugs for your diesel engine.
Frequently Asked Questions
Can you drive with a faulty glow plug?
Yes, most diesel cars can still be driven with a faulty glow plug, especially once the engine is warm. However, it is not recommended. Driving with faulty glow plugs can lead to poor combustion, increased exhaust smoke, reduced fuel efficiency, and over time, potential DPF (Diesel Particulate Filter) problems.
How much does glow plug replacement cost in the UK?
Glow plug replacement in the UK typically costs between £100 and £250. The final price depends on the number of cylinders, engine accessibility, labour rates, and whether premium glow plugs are used. Independent garages are usually cheaper than main dealers.
Should all glow plugs be replaced at the same time?
Yes. If one glow plug has failed, the others are often close to the end of their lifespan. Replacing the full set at once reduces labour costs and ensures balanced engine performance, especially during cold starts.
Do faulty glow plugs cause an MOT failure?
Glow plugs do not directly cause an MOT failure, but they can indirectly lead to one. Faulty glow plugs may cause excessive smoke, poor cold starting, or emissions-related issues, which can result in a failed MOT test.
How long do glow plugs usually last?
Most glow plugs last between 60,000 and 100,000 miles. Their lifespan depends on driving style, climate, engine type, and how often the vehicle is started from cold.
What happens if a glow plug snaps during replacement?
If a glow plug snaps inside the cylinder head, removal becomes more complex and expensive. Special extractor tools may be required, and in severe cases, cylinder head removal may be necessary. This is why professional replacement is often recommended on higher-mileage engines.
