What Controls When Case Fans Spin?

[classicsoftware]

Looking at a system that is driving me crazy. BSOD with and error about a file associated with the network card. Take the system apart to replace card and I notice one of the case fans does not always turn on. If you give it a shove it spins fine. There does not appear to be any resistance, so I'm here with a question.

Is it the BIOS or the OS or both that controls when the fan spins up. Is the fan going bad? It makes no noise when it does spin to indicate it is failing. Is it the motherboard?

Looking for some guidance.....

[123456]

I think the BIOS controls it via temperature readings. Does the fan have a 3 or 4 pin connector? OS has nothing to do with controlling the fan speed.

[mjc]

It depends on where the fan is connected...if it is connected to a header on the motherboard, then yes it is probably under BIOS control. If it is connected to a 'drive' power connector then it should always be on (unless it has its own thermal controls...but those types of fans aren't very common).

[classicsoftware]

It's a three pin header on the motherboard. I would think the fan should either run at start-up or stop if I manually start it based on the temps as the sensors report them.

[jlreich]

Try another header, or an adapter for a molex connector. If it is controlled by the BIOS it wouldn't keep spinning when you get it started by pushing it. Either the header is going bad or the fan is.