I have an HP Pavilion and I am getting a message right after turning the unit on that the CPU fan is not working and that it will power itself off in 5 seconds. When I open it up, the CPU fan is working, but the chassis fan is not. The chassis fan appears to be OK since I can plug it into the CPU fan connector and it runs.

Is there are way that I can trick the MB or BIOS or whatever it is that is saying that there is no fan, so that the computer will boot?

More details:
The fans have 3 wires. On the chassis fan connector without the fan attached, the pin that goes to the blue wire is shorted to ground, as is the black wire's pin. The red wire's pin starts at 12V right after power on, but quickly goes to 0V.

I tried connecting the blue wires of the chassis fan and the CPU fan to the connector of the CPU fan, and the CPU fan continues to turn but the chassis fan does not. I am guessing because the chassis fan controller doesn't detect any rotation and shuts off the power to the fan. I could connect the two fans to the one cpu fan connector, and they would probably both run, but I am guessing that the unit would still shut down since nothing is running off the other fan connector.

It just seems a waste to throw away the MB just because of the lousy fan rotation sensor. Maybe I just need to quit whining in cough up the money...

sounds to me like there is a fault somewhere in that particular circuit. If it was working correctly, you could simply properly connect a fan to it and all should be good.

My mainboard has an option "SYS FAN Control" in BIOS that allows me to turn on or off the system fan. Maybe your computer has similar options?