MB Medic

My aux fan turns on when i turn the key but before i start the engine. Sometimes it does it, sometimes it doesn't. It's weird that it doesn't always do that, do you have any explanations for that?

Should i test the blue 2 pole temp. switch first? Or do i have to test the red switch first?

Assumming it's the blue 2 pole temp. switch, how much is it? is it a difficult thing to do myself? How do i go about changing it?

Thanks in advance.