Briefly on at start-up it is normal — the glow plugs are warming. If it stays on or flashes, the ECU has logged a fault.
Here's a quick, honest answer from a working UK mechanic — what causes it, what it costs to fix, and what to do next.
Can I drive with it flashing? — Drive gently to a garage. A flashing light on a diesel often means a stored emissions or injector fault that needs prompt attention.
Will it cause an MOT fail? — Yes if it remains illuminated after start-up — same rule as the engine management light.
Bob's Mechanical Repairs — independent family-run garage in Birnam, Dunkeld, Perthshire. Call 01350 727 276 or email bob@bobsmechanicalrepairs.co.uk.