Often the most difficult step when building or upgrading a gaming PC is connecting all the power cables.
A CPU fan’s connector can be plugged into either the CPU_FAN or CPU_OPT header on your motherboard—but choose wisely, because having it plugged into the wrong one can lead to your system not booting and your BIOS throwing out errors.
The long story short is that you must have a CPU fan plugged into the CPU_FAN header, otherwise your motherboard likely won’t register that there’s a CPU fan connected at all.
It’s usually only when you need to connect multiple CPU fans that you should consider using the CPU_OPT header.
What is CPU_FAN?
A motherboard’s 4-pin CPU_FAN header is its primary header for plugging in your CPU cooler’s fan, and it’s usually located near the CPU socket. Your motherboard should register this CPU_FAN header as the primary one for CPU cooling. This means if there’s no fan connected to the CPU_FAN header, your motherboard will likely think there’s no fan cooling your CPU at all and will prevent you from booting the system.
Because of this, it’s crucial that you have your CPU’s fan plugged into the CPU_FAN header. This lets your motherboard see that the CPU is being cooled, and it lets you configure and regulate the CPU’s temperature by adjusting CPU fan speed in the BIOS.
What is CPU_OPT?
A motherboard’s 4-pin CPU_OPT (‘CPU Optional’) header is its optional header for plugging in CPU fans. If your CPU cooler uses two different headers, you can plug the main one into your CPU_FAN header and the other one into your CPU_OPT header. If there are more than two headers, you can use a splitter to combine them and plug the secondary ones into the CPU_OPT header with one cable.
Your system should boot without anything plugged into CPU_OPT, and its primary use is for connecting cooling systems with multiple fans, or for powering AIO cooling pumps when you can’t use the AIO_PUMP header on the motherboard. Just be aware that you won’t be able to control the AIO pump’s speed in the BIOS if it’s plugged into CPU_OPT.
CPU Fan Header Locations
Both CPU_FAN and CPU_OPT headers should be located close to your motherboard’s CPU socket. They’re often found side-by-side, up and to the right of the CPU, somewhere near the DIMM slots for RAM. Your motherboard should have ‘CPU_FAN’ and ‘CPU_OPT’ written right next to these headers.
Their locations can vary depending on the motherboard, however. If you can’t find the headers you should check your motherboard’s manual which should have them clearly labelled on a diagram.
Can You Use CPU_OPT for Case Fans?
You can use the CPU_OPT header for case fans, but it’s not recommended when there are SYS_FAN (‘system fan’) or CHA_FAN (‘chassis fan’) headers available. Just like CPU_FAN, CPU_OPT is linked only to the CPU’s temperature sensor, meaning it’s not linked to overall system and other component temperatures. As such, if you plug a case fan into the CPU_OPT header, you won’t be able to regulate its speed based on the overall system temperature—instead, it will only be regulated by the CPU’s temperature.
However, if you have no more SYS_FAN or CHA_FAN headers free on your motherboard and you need to plug in another case fan, you can plug it into CPU_OPT.
CPU_OPT vs CPU_FAN
Both CPU_OPT and CPU_FAN headers can be used for CPU fans, but, as the names suggest, only one of these is ‘optional’. There must be a fan plugged into the CPU_FAN header, otherwise your motherboard will likely think there’s no fan cooling the CPU and it won’t let you boot.
CPU_OPT should be used for secondary CPU cooling connectors, such as you might find in a multi-fan radiator setup. It can also be used for an AIO’s pump if you can’t connect it to the correct AIO_PUMP header, but this will mean you can’t control the pump speed from the BIOS. Finally, you can plug case fans into CPU_OPT if there are no SYS_FAN or CHA_FAN headers available, but these fans’ speeds will then only be regulated based on the CPU’s temperature, and not the system’s overall temperature.