# Diesel glow plugs: fault symptoms and a flashing glow plug light _Updated: 8 November 2026_ **Short answer:** When diesel glow plugs fail, the car starts hard from cold, the engine runs unevenly after starting and smoke comes from the exhaust. The glow plug light, a yellow coil on the dashboard, normally stays on only while the plugs heat before starting; if it is on or flashes while the engine runs, drive slowly and get it checked as soon as possible. > **The service we recommend: Diagnostics** We read faults from every computer in the car and tell you what is urgent. We tell you the price when you tell us the model. [More about the service](https://autoindustrija.rs/en/services/car-diagnostics) ## What glow plugs do and when the light is normal A diesel has one small electric heater, a glow plug, in each cylinder. Before starting, the glow plugs warm the place where the fuel burns, so a cold engine starts more easily. A diesel has no spark plugs like a petrol engine. The fuel ignites only from the heat of the air the engine squeezes hard. When the engine is cold, the cold metal of the engine takes part of that heat, so without glow plugs the engine starts hard. Turn the key to the position where the dashboard lights come on, but not all the way. The glow plug light is on while the glow plugs heat. When it goes out, turn the key all the way and start the engine. The engine computer decides how long they heat, by the engine temperature: longer in frost, and briefly or not at all on a warm engine. Many newer cars start heating as soon as you unlock them or open the door, so the wait before starting is shorter. ## The glow plug light is on or flashes while driving When the glow plug light is on or flashes while the engine is running, something in the glow plugs or in the engine is not working properly. It can mean the glow plugs need replacing. On a Volkswagen diesel the same light is also the engine light, so it is on or flashes when the fault is in another part of the engine too. The car can then go into a safe mode and have less power, to protect the engine. If you keep driving as if nothing were wrong, the engine can be damaged. On newer diesels the computer monitors the glow plug in each cylinder, so the fault says which cylinder's plug isn't working or that the module that switches them on is faulty. ## Symptoms of faulty glow plugs When the glow plugs do not work, a cold engine lacks the heat to burn the fuel well. This is how it shows: - **The car starts hard from cold:** This is the most common sign. Without the heat of the glow plugs, a cold cylinder may not reach the temperature at which the fuel ignites. - **The engine runs unevenly or misfires after starting:** In the cylinder with a bad glow plug the fuel does not ignite properly. - **White smoke comes from the exhaust:** This is fuel that did not burn, because the cylinder is not hot enough. A faulty glow plug module also causes white smoke. - **Black smoke comes from the exhaust:** The engine gets too much fuel for the air it has. Bad glow plugs are one possible cause. - **The car uses more fuel:** Without enough heat the engine runs weaker and has to work harder. Smoke and uneven running can have another cause in the engine too, so a part is replaced only after a check. [More about why a car won't start](https://autoindustrija.rs/en/advice/car-wont-start) ## What happens if you drive with bad glow plugs Bad glow plugs do more harm than hard starting, because glow plugs do not work only at start. After starting they keep heating for several minutes, until the engine warms up, so the fuel burns completely and the engine pollutes less. On newer diesels they also help the DPF filter in the exhaust, which traps soot, clean itself. When the glow plugs do not work properly: - in winter the car may not start at all - the soot filter cannot clean itself, so it clogs, and that is an expensive repair - the EGR valve, which sends part of the exhaust gases back into the engine, gets dirty faster - the engine misfires, uses more fuel and pollutes more That is why we replace faulty glow plugs before they damage the filter. When diagnostics points to a glow plug or the glow plug module, we repair or replace it. [More about the auto electrician](https://autoindustrija.rs/en/services/auto-electrician) ## The soot filter light is on together with the glow plug light The DPF filter light means the filter is filling with soot or there is a fault in the filter system. When it is on, about 10 minutes of driving faster than 65 km/h helps. [More about DPF regeneration](https://autoindustrija.rs/en/advice/dpf-regeneration) If the filter light stays on after that drive, turns red or another filter light comes on, don't wait. The damage grows and the repair becomes much more expensive. [More about DPF cleaning](https://autoindustrija.rs/en/services/dpf-cleaning) ## How we check and replace glow plugs Measuring shows which glow plug is weak and which works, and whether the module is at fault. A glow plug can snap while being unscrewed, so it is soaked in oil first and undone with a torque wrench. A stuck plug is never forced: a snapped one is removed with a special tool. ## What the price of glow plug replacement depends on **The price of replacing glow plugs depends on** - **The number of cylinders:** each has its own glow plug. - **The type:** metal or ceramic, for newer engines. - Whether a plug is stuck or has snapped. - Whether the fault is in the glow plug module, not the plugs. ## Frequently asked questions ### How long do diesel glow plugs last? On average, glow plugs last about 160,000 km. They can fail earlier, for example when an injector leaks and the glow plug overheats, or when a faulty module heats it harder than it should. ### Do you replace only the glow plug that failed, or all of them? When one glow plug fails, the others are usually close to the end too. That is why we advise replacing all of them at once. This way all cylinders get the same heat and the car does not need another replacement soon after. ### Does this light exist on a petrol car? No. A petrol engine starts with spark plugs and has no glow plugs, so it has no such light. Have a question about your car? Message us on WhatsApp or call 067 774 1606. ## Read also - [Car dashboard warning lights: meaning and when to stop](https://autoindustrija.rs/en/advice/dashboard-warning-lights) - [Check engine light (engine warning light): what it means and what to do](https://autoindustrija.rs/en/advice/check-engine-light) - [Car won't start: what to check first](https://autoindustrija.rs/en/advice/car-wont-start) --- Page on the website: https://autoindustrija.rs/en/advice/diesel-glow-plug-light auto-industrija, Belgrade. Phone and WhatsApp: +381677741606. E-mail: kontakt@autoindustrija.rs.