Hi,
I'm currently turning an old HP Compaq dc7800 SFF into a "silent" HTPC. To do that, I want to change the chassi fan and the PSU fan to PWM fans from Arctic Cooling, so far so good. I also want to completely remove the CPU fan, which is not needed (The thermally controlled chassi fan blows through a duct on the underclocked E6750 which stays amazingly cool, while the GPU, HD5450, does all the hard work without breaking a sweat ).

Now, here's the problem: The BIOS wont boot without all fans being present. How do I fake a fan being present?
All the fan headers are 4-pin PWM sockets. Can I just cut the connector from one of the old fans and place it on the CPU fan socket? Or will I break something?

EDIT: Forgot to mention, I haven't found an option to override the fan check in BIOS control.

My last option is of course to replace the CPU fan as well, but as it's not needed I really don't want to do this...

Any suggestions appreciated!

If the fans you are using puts out an RPM signal then you could feed that to the CPU fan header. Many PSU fans does that - the fan is powered by the PSU but provides a 3pin connector with only ground and RPM connected so one can connect that to a fan header on the motherboard to be able to monitor the RPM of the PSU fan.

Or you could build a RPM signal generator. If I'm not mistaken they are not that complicated to build if you have some electronics skill (can handle a soldering iron and such).

Oh, I didn't realize that the fans are detected using the RPM signal. Sorry for that..

Since the Arctic Cooling PWM-fans use some kind of strange daisy chain wiring, maybe I can just plug the extra 3-pin RPM plug to the CPU fan socket?

On the left: 3 pin plug to send fan RPM signal to motherboard

Is that in fact what they are trying to say in the installation instructions, where it says "If there is an extra 4-pin PWM socket on the mainboard".

The fans, though ordered, haven't been sent yet so I'm not able to test it just yet.
Sorry if I'm asking really stupid questions!

Yes, that should be the solution to your problem. Arctic provides each of the shared PWM fans with a separate rpm sense cable/plug so that you can select which the fans you want to be monitored by the CPU fan socket. Normally you would plug the yellow cable from the chosen fan into the last socket on the daisy chain. But given your scenario it should work if you plug it directly into the CPU socket.