Opel Astra heater

Opel Astra 2.0 DTI 16V Year 2000

My problem is that the warmthrie witness does not turn on as long as it does not turn on, but when it turns on it starts well, it does not matter how cold it is heat.

I have changed the 4 heater and remain the same.
My question is who tells the witness to turn on or what isn't it?
It is a rest or a tª probe or what?

Thank you very much in advance

What sends the signal for the prequalge to the heater is an ambient temperature probe, (I think the same one that indicates the temperature on the screen, but I am not sure).

Anyway, 2.0 DTI and 2.2 DTI do not light the heaters until there are less than 0ºC in the street.

I am not being able to be more help.