Key statistics. Approximately 28.38% of initial MOT tests carried out in Great Britain during 2022/2023 resulted in failure. (DVSA, 2022/2023). Issues related to 'Noise, emissions and leaks' accounted for 3% of MOT failures in 2024-2025, which faulty glow plugs can contribute to. (Green Flag / DVLA, 2024-2025).
A flashing glow plug warning light on your dashboard, often resembling a coiled filament, typically indicates a fault within your diesel engine's glow plug system. This could mean one or more glow plugs are failing, there's an issue with the glow plug relay, or a problem with the wiring, which can impact your car's starting ability and emissions performance.
What exactly are glow plugs and what do they do?
Glow plugs are essential heating devices found exclusively in diesel engines, designed to aid cold starting by pre-heating the air in the combustion chamber. When you turn the ignition, these small, pencil-shaped components rapidly heat up, creating the necessary warmth to ignite the diesel fuel, especially in colder weather. Without them functioning correctly, your diesel car will struggle to start, particularly on a chilly morning, as diesel engines rely on compression ignition rather than spark plugs.
Why is my glow plug warning light flashing?
A flashing glow plug warning light usually signifies a detected fault within the glow plug system, indicating an urgent issue that requires attention. This isn't just a minor glitch, but a signal from your car's Engine Control Unit (ECU) that one or more glow plugs might be faulty, the glow plug control unit is malfunctioning, or there's an electrical problem in the circuit. Sometimes, it can even point to issues with the fuel injection system or a DPF (Diesel Particulate Filter) problem, as glow plugs can assist in DPF regeneration.
What are the common symptoms of faulty glow plugs?
Beyond the flashing warning light, faulty glow plugs can manifest in several noticeable ways that impact your driving experience. You might experience difficulty starting your engine, especially in cold weather, requiring multiple attempts before it finally splutters to life. Other common symptoms include a rough idle or misfires immediately after starting, reduced engine performance, and an increase in white smoke from the exhaust during startup as unburnt fuel is expelled. You might also notice your car shaking when idle or a decrease in fuel efficiency.
Can I drive with a flashing glow plug warning light?
While you might be able to drive your car with a flashing glow plug warning light, it's
Frequently Asked Questions
Can I drive with the glow plug warning light flashing?
While you might be able to drive, it is not recommended. Continued driving with a faulty glow plug system can worsen emissions issues, cause DPF problems, lead to engine misfires, and decrease fuel efficiency.
How much does it cost to replace glow plugs in the UK?
In the UK, the average cost for glow plug replacement typically ranges from £170 to £250, including parts and labour. However, this can vary significantly based on your vehicle's make, model, and the garage chosen.
Will a flashing glow plug light cause an MOT failure?
A flashing or continuously illuminated glow plug warning light can lead to an MOT failure. Specifically, if the light indicates persistent engine faults or emissions-related issues (such as excessive smoke), your vehicle may fail the MOT test.
