How do glow plugs work?
Glow plugs are tiny electric heaters fitted to diesel engines (one per cylinder). Before and just after you start, they heat the tip of the plug and the air/fuel around it, to help the diesel ignite cleanly when the engine is cold. On older cars you’d wait for the amber coil light to go out; on most modern diesels the pre-heat is so fast you barely notice it, and the plugs may stay on briefly after start (“after-glow”) to smooth running and cut white smoke. They’re not the same as spark plugs (petrol engines use sparks; diesels mostly ignite fuel by compression and heat).
How do I know I need glow plugs replacement?
You will notice some clear signs and symptoms, especially during colder weather:
- Hard starting when cold: the engine cranks for longer than usual, then finally catches.
- Rough idle for the first 10–30 seconds: it shakes or sounds uneven, then smooths out as it warms.
- White/grey diesel smoke right after start: fades quickly once the engine’s warm.
- Glow-plug warning light misbehaving: stays on much longer than normal, flashes, or comes back on after the engine starts.
- Problem is temperature-dependent: starts are OK when the engine is warm, but poor after it’s been parked overnight.
If you’re getting the cold-start as well as the brief smoke/roughness combo (and the light misbehaves), you’d probably need to book “glow plug replacement” or a diagnostic check if you’re not sure. Many mechanics will test each plug and replace the failed ones; on higher-mileage cars with easy access, it can be sensible to replace the set to avoid repeat labour.
How to test glow plugs?
Glow-plug testing and removal can be tricky, plugs often seize in the cylinder head and can snap; if you’re not fully confident or a plug feels tight, stop and let a professional handle it to avoid costly head repairs. You can also book a diagnostic inspection if you’re not sure what needs replacing
This is steps to do a basic multimeter test at home:
What you need: a digital multimeter.
- Switch ignition OFF, engine cold. Pop off the plastic engine cover.
- Unplug the small connector from the top of one glow plug.
- Set meter to low ohms (Ω). Black probe on clean engine metal (ground), red probe on the plug’s top terminal.
- Read the resistance: Around 0.5–2 Ω means likely good. OL / very high → open circuit means failed plug.
- Near 0 Ω (short) means also bad.
- Repeat for every cylinder. One odd reading is your suspect; several bad ones suggest age or a control issue.
If you're not sure, it's best to leave it to the professionals and book a diagnostic inspection if you suspect something is wrong with the glow plugs.
How to fix glow plug warning light?
The glow plug light should come on briefly before a cold start, that’s normal pre-heating. If it stays on after the engine starts, comes back on, or flashes while driving, the car is telling you there’s a fault in the glow-plug system or a related control circuit. If the light is steady and the car runs normally, you can usually drive gently to a garage the same day. If it’s flashing, the car feels down on power, or you see heavy smoke/misfires, treat it as urgent and avoid long trips.
A proper fix will likely start with a diagnostic scan to see which cylinder or circuit is flagged. A technician will then test each glow plug, check that they’re actually getting power during pre-heat, and inspect the fuse, relay/controller, wiring and battery/charging. Once the fault is identified, they’ll replace the failed part, clear the codes and confirm a clean cold start. One caution: glow plugs can seize in the cylinder head forcing them can snap the tip so removal is best left to a pro.
If you're not sure, book a warning light diagnosis; in many cases the fix is a glow plug replacement (sometimes the full set), followed by a quick re-test to make sure the light stays off.
Can you drive with glow plug light on?
Usually yes, for a short, gentle trip to a garage but it depends on how the car behaves.
The glow plug light should only show briefly before a cold start. If it stays on after starting yet the car drives normally, you can typically drive carefully to a nearby workshop the same day. You can expect longer cold cranking, a lumpy idle for a few seconds, and maybe a puff of white/grey smoke, then it often smooths out once warm. Avoid long journeys until it’s checked.
If the light is flashing, the engine feels down on power, you see heavy smoke or misfiring, or another warning lamp appears, treat it as urgent: limit driving and book diagnosis immediately (some cars use the glow-plug lamp to flag wider engine/emissions faults).
Can a mobile mechanic carry out a glow plug replacement?
Yes. Glow plugs are small, and on many diesels they’re accessible enough for a mobile mechanic to diagnose and replace on your driveway. By choosing a mobile mechanic, you can enjoy a hassle-free and convenient experience, saving both time and money that would otherwise be spent driving your vehicle to a traditional garage. ClickMechanic also offers free collection and delivery if your vehicle is drivable.
Top Mechanics For Glow Plugs Replacement & Repairs
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Garage in Bristol, 20 years of experience
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JAMIE • 27th October 2025
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Garage in Aylesbury , 20 years of experience
Main dealer work- 5 years ZM Car repairs Milton Keynes- 8 Years TWK Limited - Specialist in house mechanic 7 years
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Garage in Birmingham , 9 years of experience
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Garage in London, 15 years of experience
Previously worked at main dealers before such as Toyota, BMW, Peugeot Citroen, Volkswagen Group. Work on all models of vehicles have all the appropriate tools and diagnostics for them all.
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Garage in Ashington, 8 years of experience
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Garage in Uxbridge , 10 years of experience
All garage-based experience from apprentice to my own place. I have many clients who always recommend me to their friends and family. I am able to do all work in house, up to now theres nothing I havent ...
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ALISTAIR • 15th October 2025
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TONY • 8th October 2025
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Garage in Bedford, 16 years of experience
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Mobile mechanic in Stoke-On-Trent, 5 years of experience
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Mobile mechanic in Swindon, 15 years of experience
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Frequently asked questions about glow plugs replacement & repairs
Read our answers to questions about glow plugs replacement costs, symptoms and more.
What do glow plugs do?
Glow plugs were originally used to assist with starting diesel engines in cold temperatures when the heavy oil used to coagulate. Now that diesel has many additives to prevent this, glow plugs have taken on the role of heating the incoming fuel / air mix to provide greater efficiency while the engine warms up. This also assists with decreasing the noise during cold running.
What does a glow plugs replacement involve?
- The mechanic will pre-soak the glow plug threads with penetrating oil or similar and wait for it to take effect.
- Mechanic will then ease the glow plugs undone and remove them one by one
- New glow plugs will then be installed
- Engine start up will be tested and error codes cleared down
Can I change glow plugs myself?
Unless you are a qualified mechanic with the required skills, training and equipment to do the job, we strongly recommend a glow plugs replacement is only carried out by a professional mechanic. ClickMechanic can help get you a vetted and local mechanic to carry out the work. Just enter your postcode and your car's registration number at the top of this page and select the work needed to get an instant fixed price up front.
What are the symptoms of bad glow plugs?
You may experience some of the following symptoms
- Glow plug warning light flashing when the ignition is switched on
- Engine management warning light (EML) lit
- Struggling to start from cold
- Excessive fuel consumption while the engine warms up
How long will a glow plugs replacement take?
On average, glow plugs replacement times can range from 1 to 2 hours with an average of just over 1 hours. The time that the job takes will depend on a number of factors, like your vehicle's make and model, year, as well as other factors. Select your car at the top of this page for information on job time and cost for your vehicle through ClickMechanic.
How often should glow plugs be replaced?
The majority of diesel vehicles will go through their life without requiring them to be changed, however like all electrical units, they can fail. If you are experiencing any of the listed symptoms, ideally would have the glow plugs changed as soon as possible. If the engine is starting OK, though and there is no EML lit, then get the glow plugs changed when you are able to.
Can you drive with a broken glow plug?
If you suspect there is any issue with the glow plug or there are concerns about its age or condition, book an inspection with a professional mechanic with immediate effect. If the glow plug is physically broken or snapped do not drive the vehicle as this will lead to severe engine damage.
What is the Glow Plug Warning Light?
The glow plug warning light provides information on whether the glow plug has warmed up, and whether there is a problem with it. When the ignition is switched on the glow plug warning will come on as the glow plugs through it's pre-heat cycle, the light will go off again once the glow plugs have heated up.
If the glow plug warning light does not go off it means that the engine is not getting the right amount of heat from the plugs, which can affect starting and performance. If the glow plug warning light is flashing it means that the glow plugs are not getting hot enough to start the car and that there is a problem. This can be caused by a number of things.
Is it safe to drive with the Glow Plug Warning light on?
Generally it is ok to continue to drive at slower speeds with a flashing glow plug warning light, however, it does entirely depend on the scale of the glow plug issue. Often the engine also goes into 'limp mode' and power will be limited. It is highly recommended to get it checked by a qualified mechanic as soon as possible as it could lead to further problems and engine damage.
In rare cases the engine warning light may also flash simultaneously with the glow plug warning light, which can indicate an overheating engine. When this happens, the best thing to do is stop as soon as it is safe to do so and seek help from a breakdown service or a qualified vehicle technician
Will my car fail the MOT test if the Glow Plug Warning light is on?
Yes, it is possible that a flashing glow plug warning light may lead to an MOT fail. The underlying issue may lead to inefficient running of the engine and an emissions issue, on which a vehicle may be failed.
Is a faulty glow plug an MOT failure?
Yes, a faulty glow plug may result in an MOT failure - if the glow plug warning light on the dashboard goes off after a few seconds then it will normally pass the MOT (this is a sign the plugs have heated up correctly). If the warning light continues to flash or illuminate on the dashboard, it could result in an MOT failure as your vehicle may experience emissions issues.
How long do glow plugs last?
You can expect glow plugs to last at least 100,000 miles but they can become faulty at an earlier stage so may need replacing accordingly.
Are glow plugs easy to replace?
Glow plugs can be very hard to replace, especially if they are located in hard to reach location or in the case they are seized or stuck. The job best left to a certified mechanic as it needs the right parts and equipment to ensure the replacement is done properly. If a glow plug breaks on removal, an bits drop into the inside of the engine this will lead to the engine having to be opened up. Under no circumstances should the vehicle then be started as it can lead to catastrophic engine damage.
How many glow plugs do I need?
You need one glow plug for each cylinder in your engine so a 4 cylinder diesel engine will need 4 glow plugs.
How do you know if your glow plugs need changing?
Some tell-tale signs that your glow plugs may need changing include:
- Engine misfiring
- Rough idling
- Decrease in fuel efficiency
- Difficulty starting the car (especially in cold weather)
- Dark or black-coloured exhaust smoke
How much does it cost to replace the glow plugs?
A glow plugs replacement generally costs between £125 and £250 with an average price of £170. Ultimately, the cost of replacing glow plugs depends on your car, what parts your car's manufacturer recommends using for the replacement, and how many are needed. Find out what you could save by having your glow plugs replaced by ClickMechanic by selecting your car at the top of this page!
Check out the ClickMechanic price for a glow plugs replacement. See the averages for different makes below. We consolidate industry standard data with millions of data points to give you the most accurate price. These estimates may include Glow plug.
| Name | Average Cost | |
|---|---|---|
| Audi | £188 | Get exact price |
| BMW | £285 | Get exact price |
| Citroen | £260 | Get exact price |
| Ford | £295 | Get exact price |
| Land Rover | £313 | Get exact price |
| Mercedes-Benz | £211 | Get exact price |
| Nissan | £177 | Get exact price |
| Peugeot | £263 | Get exact price |
| Renault | £182 | Get exact price |
| Toyota | £192 | Get exact price |
| Vauxhall | £187 | Get exact price |
| Volkswagen | £197 | Get exact price |
Estimates on average repair costs, such as the above results for a glow plugs replacement, are based on ClickMechanic's quote data which combines multiple third-party sources. Actual costs may vary. We provide estimates on average repair costs for information only and accept no liability for any inaccuracies or errors. If you think an estimate is incorrect, please contact us to let us know.
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Mechanics that undertake glow plugs work
- Adam’s Autocare, Burntisland
- Autolink Service Centre, Wolverhampton
- Automerc Ltd, Milton Keynes
- Autostop Stratford Ltd, Stratford-Upon-Avon
- BB Automotive, Biggin Hill
- Best Motor Company Services, Rainham
- Bishopbriggs Mot Centre, Glasgow
- Boni's, Sunbury-On-Thames
- C G Mechanics LTD, West Malling
- LEE MCCARTHY, Kent
- Mem's Mobile Mechanic, Sidcup
- Morgan Autos, England