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 coldThis 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 startingIn the cylinder with a bad glow plug the fuel does not ignite properly.
- White smoke comes from the exhaustThis 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 exhaustThe engine gets too much fuel for the air it has. Bad glow plugs are one possible cause.
- The car uses more fuelWithout 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
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
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
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
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.
