I have Audi A4 B7 2005 2.5TDI.
The temperature drops below 0 °C where I live.
When it's cold (e.g. starting from 5 °C) my car starts having problems starting up.
When I start it in the morning everything is OK.
The glow plugs turn on and the car start's fine whatever temperature it is outside.
After I stop the car and let it cool for 1-2h and try to start it again it has problems.
The glow plugs don't turn on and it takes as while before the engine can crank itself up.
I know that my fuel injector's aren't in the best shape. I have had them tested.

1. Should the glow plugs turn on for a few seconds if the car has been idle for 2 hours (with temperatures below 0)? If yes, then can it be that I have a faulty sensor and it only turns the glow plugs on when the car has cooled down a lot?
2. If no then is it possible to change the temperature at which the glow plugs turn on? Or always have them on for certain amount of time.

Thank you for any kind of feedback!