Good afternoon.
The first thing I wanted to comment is that you apparently commenting different things.
Sergio1671 It seems to me for what I am leelland refers to the ignition heating calling them spark plugs, it gives me this impression because in the car model you also put that it is a 1.9 d that is to say that being diesel does not carry spark plugs by the system of compression and explosion of the diesel engines that effectively as you say they only work at the time of turning on the engine. And Sosa10 is explaining the breakdown in ignition spark plugs.
Sergio1671 What are you referring to?
If you put 3 cylinders as you say almost certainly it is because of some diesel injector. What kind of injectors do you have? It would be good to look at them too ...
By the way, how did you do the check with the lamp? Because it seems to me that what you looked only was that the heater was fine and that he made a good dough. M also seems very rare that the air intake goes so down, it must go for the part of the grill put k it is where the air is best, but I am wrong to the side of the good left lighthouse when you can look at these things and clarify to me a little also that I stay with the doubts that I already comment before.
I hope to help you in the future ..
All the best