Asus M4N78 SE - Manual (Page 18)

1-8 Chapter 1: Product introduction
7. Connect the CPU fan cable to the CPU_FAN connector on the motherboard.
Do not forget to connect the
CPU fan connector! Hardware
monitoring errors can occur if
you fail to plug this connector.
5. When the CPU is in place, push down the
socket lever to secure the CPU. The lever
clicks on the side tab to indicate that it is
locked.
6. Install a CPU heatsink and fan following
the instructions that came with the heatsink
package.