Diesel Diagnostics Swindon, Independent Specialists
Glow plug light flashing? Here is what it means
A glow plug light that flashes means something different to one that simply stays on, and the difference matters. This article covers what each one is telling you, why the fault is often nothing to do with the glow plugs, and what to do next.
It applies to diesel Volkswagen, Audi, SEAT, Skoda and Cupra models, which is what we work on at VGS Performance in Swindon.
What does a flashing glow plug light mean?
A flashing glow plug warning light indicates a serious engine fault, often unrelated to the glow plugs themselves, and means the ECU may have placed the engine into a reduced power mode. On a diesel it effectively does the job the engine management light does on a petrol car.
This is the part that catches owners out. The symbol suggests glow plugs, so people replace them, and the light comes straight back because the fault was somewhere else entirely.
What does a steady glow plug light mean?
A steady glow plug light during start up is completely normal. It shows the glow plugs are pre heating the combustion chambers, and it goes out when they are ready and the engine can be started.
The light staying on after start up, or coming on while driving, is not normal and should be investigated.
What causes a flashing glow plug light?
Because it functions as a general engine fault warning on a diesel, the causes cover a wide range. These are the ones we see most often.
Only the last of those actually involves the glow plugs, which is why reading the stored code before replacing anything saves both time and money.
Glow plug light flashing and the car feeling flat? Call VGS Performance in Swindon on 01793 228485 rather than guessing at it.
Can you keep driving with a flashing glow plug light?
Drive only as far as you need to in order to get the car somewhere safe, and book it in promptly. A flashing light usually means the engine has gone into a protective mode, so you will often notice reduced power alongside it.
Continuing to drive in that state will not usually cause immediate damage, but the underlying fault is still there and some of the causes get more expensive the longer they are left. DPF problems in particular do not improve on their own.
How do you diagnose a glow plug light fault?
The stored fault code is the starting point, and live data tells you what the code actually means on your car. Because the light covers such a broad range of causes, diagnosing it from the symbol alone is not possible.
We read every module with dealer level equipment, which on a diesel with this light typically means looking at fuel system pressures, boost figures and DPF status rather than starting with the glow plugs.
FAQs
Should I just replace the glow plugs?
Not without reading the code first. A flashing glow plug light is usually caused by something other than the glow plugs, so replacing them often changes nothing and leaves you with the original fault plus a bill.
Why does the light only come on when the engine is cold?
A steady light on a cold start is normal, since the glow plugs are pre heating the combustion chambers. If it flashes on a cold start and then clears, there is still a stored code worth reading.
Is a glow plug light an MOT failure?
It can be. An illuminated malfunction light for a system covered by the test can be recorded as a defect, and on a diesel this light often accompanies an emissions related fault. Get it checked before the test.
Need your diesel looked at?
VGS Performance are independent Volkswagen, Audi, SEAT, Skoda and Cupra specialists on Hillmead Drive in Swindon. Book online at vgsperformance.co.uk or call 01793 228485.
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