Asus P4B - Manual (Page 31)

ASUS P4B motherboard user guide 2-9
2.4.4 Connecting the CPU fan cable
When the fan, heatsink, and the retention mechanism are in place,
connect the CPU fan cable to the connector on the motherboard labeled
CPU_FAN.
3. Push down the locks on the retention mechanism to secure the
heatsink and fan to the module base.
Figure 2-11 Fan and Retention Mechanism Installed and Locked
Figure 2-12 CPU Fan Connector
When secure, the retention locks should point to opposite directions.
CPU Fan Connector
(CPU_FAN)
Don’t forget to connect the CPU fan connector! Hardware monitoring
errors may occur if you fail to plug this connector.