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On most diesels a flashing coil light does not just mean glow plugs: it indicates a fault stored in the engine management. It often comes together with a loss of power or limp mode.

What it means

On a cold start the light comes on for a few seconds and that is normal: it is the preheating. If it flashes after starting or while driving, the ECU has detected a fault.

Most common causes

• One or more glow plugs shorted or open-circuit
• Faulty glow plug control unit
• Faults on the turbo, EGR or boost pressure
• Injection problems: injectors out of tolerance, insufficient rail pressure
• MAF sensor or DPF differential pressure sensor
• DPF regeneration interrupted several times

What to do right away

• Do not just replace the glow plugs: in most cases the cause lies elsewhere
• If the car is also in limp mode, get a diagnosis before travelling
• Have the fault codes and the real injection and boost values read

How MasterTuning helps

We read the ECU with professional tools, interpret the real fault codes and tell you what really needs repairing, without unnecessary replacements. Diagnosis, repair and software recalibration at our centres throughout Italy.

Read more: what is ECU remapping · vehicle database · find an installer

Is a warning light on? Contact us for a diagnosis or find your nearest MasterTuning centre.

Frequently asked questions

The coil light is flashing: is it the glow plugs?

Not necessarily. Flashing indicates a fault stored in the engine management, which may involve injection, turbo, EGR or DPF.

Can you drive with the preheat light flashing?

You can reach the workshop, but if the car has lost power or is in limp mode it is best to have it checked right away.

Why does a diesel start badly when cold?

Weak glow plugs, a weak battery or insufficient injection pressure are the most frequent causes: a diagnosis tells them apart.