Asus M4A78-EM/1394 - Manual (Page 21)

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 comes with the heatsink package.
You can also refer to section 1.6.2 Installing
heatsink and fan for instructions.
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.
ASUS M4A78-EM/1394 1-9