Glow plugs and the glow plug light on a diesel
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The yellow coil-shaped light is the diesel glow plug light. If it comes on before starting and goes out after a few seconds, everything is fine: wait for it to go out, then start. A hard cold start with white smoke often means a burnt-out glow plug or relay. If the light flashes while driving, the engine computer has found a fault and the code needs reading.
What glow plugs do
A diesel ignites its fuel with the heat of compressed air. A cold engine loses that heat to the cylinder walls, so the glow plugs warm the combustion chamber before starting, and on many cars for a few minutes after, so the engine runs smoothly with less smoke.
Signs of failed glow plugs
In cold weather the engine needs long cranking, runs roughly for the first seconds and puffs white smoke. Warm starts are usually fine.
With one plug out of four gone, the car still starts above freezing. The problem shows up with the first frosts, so autumn is the time to check.
Glow plug light flashing while driving
On many diesels, including VW group cars, the engine computer uses the same light as a general engine fault warning. The fault may have nothing to do with the plugs: fuel pressure, the turbo or the DPF are common causes. The car may go into reduced power mode. Drive gently and get the code read.
How we check them
We read the codes first, as they often show which cylinder's plug has failed (P0671 is the cylinder 1 circuit, for example). Then we measure each plug and check the relay (control unit) and wiring.
Old plugs can seize in the cylinder head. We remove them carefully with the engine warm, because drilling out a snapped plug is a much bigger job. Plugs are normally replaced as a set.
Want a definite answer?
Bring the car to our workshop in Estonia for diagnostics. We read the fault codes, test the sensors and tell you what is wrong.
Frequently asked questions
How long should you wait for the glow plug light?
Until it goes out. In mild weather that is a moment, in hard frost a few seconds longer. Cranking before it goes out means the chamber isn't warm enough yet.
Can you drive with one dead glow plug?
Yes, it won't harm the engine straight away. Cold starts get harder and smokier, and the starter and battery work harder. Replace it before winter.
Should glow plugs be replaced as a set?
We recommend it. When one fails, the others are just as old, and one job is cheaper than four separate visits.
Why is the glow plug light flashing?
On many diesels a flashing light while driving means the engine computer has found an engine fault, not necessarily a glow plug fault. Get the code read.
How much does a diagnosis cost?
Diagnostics cost €45/h, the price depends on the job. The cost of new plugs depends on the engine and how easily the old ones come out.