Question : Problem: CPU Fan --System shutting Down

CPU fan stuffed so I replaced it with generic cpu fan. Would not operate. Switched fan into another plug. Fan works ok. But system now closing within 15 secs because cannot detect cpu fan. How can i enter BIOS to switch off fan detection --assuming I can switch it off. Any other suggestions to get system going.

Answer : Problem: CPU Fan --System shutting Down

It sounds like the motherboard connector for the CPU fan is bad.
 If the Tach pin on the bad connector is still viable you may be able to divert the new fans tach signal to it while using another motherboard fan connector. This  will allow the motherboard to start, thinking it has a working CPU fan.
You may want to review this question and the solution given.
http://www.experts-exchange.com/Hardware/Components/Fans_Cooling/Q_22484127.html