When a fan fails, what happens to the circuit?

[BlackFalcon]

I'm trying to determine a method of notifing myself of fan failures in my case. When the fan fails will the circuit be broken or will the fan offer less resistence or what? (I'm thinking of incorperating some digital logic into a baybus.)

 

[bigjon]

From what I've seen it opens the circuit, although I suppose it's possible that the resistance would just increase. If fans just shorted (which I suppose is possible although I've never seen it - would probably have to majorly overheat to short), it would blow a fan header.

 

[Jhhnn]

Problem is, there are multiple failure scenarios.
1. open circuit
2. locked rotor
3. short circuit or reduced resistance
Modern mobo design covers all the scenarios by using rpm sensing fans, I have no idea how to do it any other way, not to say that it is impossible.

 

[Mday]

how about putting an LED in series with the fan?

 

[BlackFalcon]

Mday, that's kinda the idea I had. However, how to wire the LED to light (or turn off) when the fan fails? The wiring would be different if there was a short, open circuit, etc.
Does anyone know what kind of signal a fan tachometer outputs?