|You haven't listed your hardware in any detail except for the motherboard & even that info is incomplete. I assume it's one of these?|
There are several 4-pin CHA_FAN headers that a 3 or 4-pin case fan can be connected to, however, there is one PWR_FAN 3-pin header that is NOT meant for case fans, it's for monitoring the power supply's fan speed. Read your manual:
"we recommend that you plug the rear chassis fan cable to the motherboard connector labeled CHA_FAN1, CHA_FAN2, CHA_FAN3"
"i bought an external fan to enhance my cooling"
Why did you feel you needed the extra cooling fan if the system had been running fine for months without it? Is your power supply (PSU) mounted at the top or bottom of the case? What's the make/model of the PSU?