what's wrong with my cooling fun and it won't stop working? please read details.

i have a corsa c 17 tdi and i have a problem with the cooling fun. when i turn on the started switch of the car,the cooling fun starts working and it never stops. I did a diagnosis test and it only showed a problem with the temperature switch so i changed it. This didn't help with anything since the cooling fun is still doing the same thing, working nonstop. Anyone know where the problem is and how i could fix it?

Also, when i start the car the rpm goes high for about 10-15 seconds and won't go low. maybe the ECU machine is not working right? Do you know where all these problems come from?

Thanks
Lefteris