Expert advice: are faulty glow plugs the reason your car won’t start?

Glow plugs

When is a flat battery not a flat battery? When it’s a problem with glow plugs. OK, that’s a bit simplistic but frequently people breakdown with what they think is a battery problem, only for it to turn out to be the glow plugs that are at fault.

The reason is that diesel engines use these heaters to help them start. In warmer weather they’re not as necessary. However, when the weather gets cold, glow plugs are vital. That’s why in November 2017, the number of call outs Green Flag attended for glow plug problems rose by 112 per cent. Read on to find out what the problem is and how you can diagnose it.

What are glow plugs?

Diesel engines rely on the heat of compression rather than spark plugs to ignite fuel in the cylinder. But in low temperatures, the cold engine block absorbs any compression heat which makes starting difficult. The glow plug provides the answer, ensuring it’s an essential part of the modern diesel engine.

Although you’ll probably never see one in action as they’re buried deep in the bowels of the engine, glow plugs look like a pen. And their tip glows, rather like a car’s cigarette lighter, hence the name. This glowing end provides extra heat in each cylinder so the fuel ignites more easily and the engine starts up. Once the engine is running, the glow plugs go back to being idle.

Glow plugs

How do you know glow plugs are working?

When you turn the diesel car’s ignition on, the glow plug symbol (below) illuminates on the dash board. After fewer than a handful of seconds this goes out, indicating that the glow plugs have done their work heating the cylinders and the engine is ready to start. On more modern diesel cars that have a starting button rather than a key, the car’s computer knows to delay starting until the glow plugs have done their thing.

When that orange squiggle goes out, the glow plugs have done their job

What happens when glow plugs fail?

You will know your glow plugs are wearing out because the orange light on the dash will shine for longer before extinguishing. And the car might struggle to start. In colder weather the symptoms might eventually appear like a flat battery. That’s because you will be cranking the engine in vain as you try to start it but you’ll be draining the battery of its charge. In the end, the battery will cry enough. But that’s the effect of the faulty glow plugs making starting hard, rather than the cause of the problem.

As most cars have four-cylinder engines, that means four glow plugs. If one glow plug fails, the engine may still start but it might run roughly initially. That’s because it will take a bit of time for the cylinder without the functioning glow plug to get up to temperature and work properly. If that’s happening it’s telling you that the glow plugs need replacing.

How long do glow plugs last?

This is a difficult question to answer but we’re looking at tens of thousands of miles at the least. The glow plug is only used during the start-up phase. That means those in a car doing a lot of short journeys will be used more and have a shorter life span in terms of miles than those in cars that do high mileages between start-ups.

Scott Wilson is vehicle and customer data insight manager for Green Flag

16 comments on “Expert advice: are faulty glow plugs the reason your car won’t start?

  1. Fagan 06/03/2019 10:46 AM

    Hi Sir. I had a catalytic converter and a glow plug warning light on the dash board of my VW Polo TDI 1.6 2011 model. The catalytic converter was removed, and a free flow exhaust system was instead. I still have the glow plgp and catalytic converter signal on the dash board. What could be the problem please

  2. Ben kalinda 29/07/2019 12:18 PM

    Thanx 4 da information on g/plugs.

  3. Sarah 09/11/2019 12:20 PM

    I’m have problems as the cold weather as come, car struggles to start up and on two occasions as dried my battery. Reading the above I’d say it was my glow plugs as I’m having same problems as stated above. But my glow plug light doesn’t stay on for long a couple seconds and it goes off. Could my problem still be the glow plugs or something else had it at the Garge they think it’s the glow plugs as well but don’t what to be spending money on something that isn’t the problem.

    • Paul 20/03/2023 9:01 PM

      You need to get some one to take it out of your e c u

  4. Brad 22/02/2020 5:28 AM

    I have a 2007 santa fe diesel that will only start if I unplug a injector. Start it then plug injector on again.
    I live in Aussie where the temps are in the high 30s. Once the car is hot its starts ok but as the engine temp drops its hard to start again.
    Is it possible its glow plug problem?

  5. Thomas Kapu 13/05/2020 9:30 PM

    My car starts from a pusher or hard start but battry is new. After few minutes of stopping it cant start and the hard start continues and is very annoying. Is it that my glow plug is bad or could be something else?

    • Philip Gunnell 31/12/2020 7:48 PM

      You could have a faulty fuel pump or an Injector issue. Get a mechanic to do some tests..

  6. Howard Jones 28/08/2020 1:05 PM

    Why has my heater coil on my dash of a focus 2015 not come on

    • Philip Gunnell 31/12/2020 6:40 PM

      Either you have a faulty Glow Plug Relay and / or the Glow Plugs are faulty..

  7. bongi 31/08/2020 10:14 AM

    I have a problem with my asv engine vw station wagon it does not start,glow plug are not burning

    • Philip Gunnell 31/12/2020 7:47 PM

      Do you have a Glow Plug light that stays on longer or does not go out?

  8. Muhammad 25/11/2020 7:47 PM

    Hi my mercedes a class amg line executive 2016 When turning the ignition on the Glow plug light comes on for a few seconds and then goes out as normal.can you tell me what is problem

    • Philip Gunnell 31/12/2020 7:45 PM

      That is normal. You probably haven’t noticed thr light come on much before as now it is colder weather. The Glow Plug Light will illuminate then go out once the Glow Plugs have reached enough heat to help start your Diesel Engine..

      • Derek Russell 14/02/2021 2:34 PM

        Can you tell me I have a Land Rover freelander and finding it hard to start I had the rac out to look at it and he said glow plugs have gone on the dashboard a fault come on saying ADC not working can tell me is it the glow plugs or something else

  9. Stephen 19/01/2023 7:03 PM

    I have a PT cruiser 2006 in this cold weather my Glow plug light won’t come on so I can’t start my car but as soon as the weather is a bit warmer if I have the ignition on it can then still take 15 to 1/2 HR then light on so can start the car

  10. julie 15/05/2023 9:38 PM

    Hi, I have AClass 180D AMG Line. Coil light on dash came on whilst driving. Noticed last few months car had a lazy start. Broke down, AA guy jump started and advised battery needed replacing. I’ve just had this done and it still won5 start? Would this mean I’ve changed battery unnecessarily and it now needs replacement glow plugs? Thanks

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