Water temperature hmm

[Gogo]

Hi, i have a problem, or i think i have a problem on my Audi A6 2010 with 2.7 140kw engine (CANA). The think is that if my climate control unit is off i can get the water temperature really fast to 90, but if the AC is on (or if it is on ECO mod, so no compressor AC but still heating) i need a lot more time to get to operating temperature. I know that the main thermostat is ok because when it gets to 90 it doesn't go back when the climate unit is off. I have no problem in summer, only on cold days.

So, the problem starts when i turn on the heating, with or without AC on (you can use heater without AC or with AC). When the temps go to 90 i start the climate unit and in seconds i can see how the water temperature is going down. It goes to 80 and then goes back to 90. But, if outside is cold like -10 than on idle, after i was driving for some time and stop, with the car hot and climate working the whole time, the water temps go down to 70 and cant go back to 90 until i start driving again.

I know that on 2.0 motors there is second thermostat and he is making these problems, but on these engines i don't have the second thermostat. Does anyone know if this is normal or where to search for error? Thank You.

[Steve Q]

Have you had a diagnostic check to see if any fault codes flag up? 

[Gogo]

i have checked for error codes but there is none. i am not even shure if this is how it should be, but i had different car before and never had something like this happening.

[Steve Q]

The heating and engine temp should be consistent when running. There could be an issue with the system and id recommend getting it into a garage to be checked 

[Gogo]

i found the problem, its the main thermostat. Thank you for your answers.