one or more of the glow plugs is toast, gain access to the plugs pull the wires of and with a multimeter check the resistances as i said one or more will be open circuit , one word of caution they (glowplugs) sometimes snap in the head and also some tend to pull the threads out as you unwind them thats why you need to test them and realy only remove the one that is faulty, to gain access remove the metal air pipe from turbo to intercooler watch out for the rubber seal turbo end, wires are only push fit but can be a bit of a b***** to remove, enjoy Regards TB

When I did this job on our old Scenic 1.9dCi, another glowplug installation known for snapping the plugs on removal, I was advised to do it with the engine warm. I did, and had no problems. It is proper heart in mouth stuff applying the initial torque to crack them loose, but take your time, and you shouldn't have any problems.

Good luck!