CPU OPT vs CPU FAN Header: What Is the Difference?

CPU FAN header controls the primary processor cooler, while CPU OPT manages an optional auxiliary cooling fan for supplemental airflow.

Dual-Fan Air Coolers (Noctua NH-D15, be quiet! Dark Rock Pro)

This is the textbook use case for CPU_OPT and arguably what the header was originally designed for. High-end tower coolers like the Noctua NH-D15 or be quiet! Dark Rock Pro 4 ship with two fans intended to run in push-pull configuration. Plug one fan into CPU_FAN and the second directly into CPU_OPT. No splitters needed, no fan hub required.

Because both headers read the same CPU thermal sensor, both fans ramp up and slow down in unison. This gives you cleaner, more predictable noise behavior than using a splitter (where both fans share one signal) or putting the second fan on a SYS_FAN header (where it would respond to a different temperature source and drift out of sync). In my experience building with dual-tower coolers on Z790 boards, the CPU_OPT + CPU_FAN combination is noticeably cleaner in BIOS fan control than any splitter solution.

Using CPU OPT for a Case Fan

Technically you can plug a case fan into CPU_OPT and it will spin. But as covered in the SYS_FAN section above, it will track CPU temperature, not chassis temperature. At browser idle, even if your case is accumulating heat from your GPU and VRMs, that case fan will stay slow because your CPU is cool.

The one situation where this is acceptable: if the case fan in question is directly blowing on the CPU VRM area. In that scenario, tying the fan’s behavior to CPU temperature actually makes sense, when the CPU is under load, the VRMs are too, and having that fan ramp up with the CPU is logical. This is a niche scenario common in smaller cases or open-air testbench builds where VRM airflow is limited. For any normal front/rear case fan, stick to SYS_FAN or CHA_FAN headers.

CPU OPT for a Pump (Custom Loop)

Running a custom water loop with a D5 or DDC pump? CPU_OPT is a workable connection point, but it comes with a caveat: by default, CPU_OPT runs on a variable PWM curve tied to CPU temperature. At idle, the BIOS may drop the duty cycle significantly, potentially running your pump slower than its minimum recommended operating speed.

The fix is straightforward: in BIOS, set the CPU_OPT header to a fixed 100% duty cycle (covered in the next section). This effectively turns CPU_OPT into a constant-on header, which is exactly what a custom loop pump needs. That said, if your board has a dedicated PUMP header, which you’ll typically find on X570, X670E, Z690, and Z790 boards in the mid-to-high tier, always use that instead. Dedicated pump headers are designed from the start to run at full voltage continuously without any PWM throttling logic.

How to Configure CPU OPT in BIOS

All three major BIOS environments give you independent control over the CPU_OPT header, though the menu names differ.

ASUS (UEFI BIOS / Fan Xpert 4): In the BIOS, navigate to Q-Fan Control under the Monitor or Advanced sections. CPU_OPT appears as a separate header alongside CPU_FAN. You can assign it a custom PWM curve, link it to a specific thermal sensor, or set a manual fixed-speed percentage. For pump use, set the mode to Full Speed or manually enter 100% across all temperature points.

MSI (Click BIOS 5/6): Go to Hardware Monitor, then Smart Fan Configuration. CPU_OPT is listed individually. MSI’s interface lets you set it to PWM or DC mode, define a fan curve with up to four temperature/speed waypoints, or set a fixed duty cycle. For AIO pump or custom loop use, set it to 100% fixed.

Gigabyte (UEFI BIOS / Smart Fan 6): Access fan control via System → Smart Fan 6. Gigabyte’s interface offers the most granular control, you can select the temperature source for CPU_OPT independently, choosing between CPU, PCH, motherboard sensor, or even a thermistor input if your board has one. For normal dual-fan cooler use, leave the source set to CPU temperature. For pump use, set mode to Full Speed.

One feature worth knowing: on some higher-end ASUS and Gigabyte boards, you can decouple CPU_OPT from CPU_FAN entirely in BIOS, assigning it a completely independent fan curve while CPU_FAN follows its own profile. This is useful if you want your AIO radiator fans to behave differently from your pump, or if you’re running a supplemental fan on CPU_OPT that doesn’t need to mirror the main cooler’s curve.

What If Your Motherboard Has No CPU OPT Header?

Budget motherboards, particularly entry-level B450, B550, H610, A320, and H510 boards, frequently omit CPU_OPT to trim the BOM cost. If yours doesn’t have one, you have several workable options:

• SYS_FAN / CHA_FAN with manual fan curve: Plug the second fan into a chassis fan header and manually configure its curve in BIOS to approximate your CPU cooler fan’s behavior. Not ideal, but functional.
• Dedicated PUMP header (if available): Some budget boards that omit CPU_OPT still include a PUMP header. For AIO setups, this is actually the better option anyway.
• Fan controller or hub: A standalone fan hub or PWM controller takes one motherboard header as input and expands it to four, six, or more outputs with individual control. This is the cleanest solution for complex builds with multiple fans needing management.

It’s worth checking your board’s manual before assuming CPU_OPT is absent, some boards label it differently (ASUS occasionally uses CPU_FAN2 on certain Z-series boards) or place it in a non-obvious location near the top edge of the PCB.

Frequently Asked Questions

What is CPU OPT?

CPU_OPT stands for CPU Optional. It is a secondary 4-pin PWM header on your motherboard that outputs the same signal as the CPU_FAN header but does not cause a POST error or system halt if left unplugged. It is designed to power an additional CPU cooling fan, an AIO pump, or any supplemental CPU-related cooling component. Not all motherboards include it, budget boards frequently omit it, while mid-range and higher boards almost always include at least one.

Should I use CPU OPT?

Yes, if you have any of the following: a dual-fan tower cooler, an AIO liquid cooler with separate radiator fans, or a custom loop where your board lacks a dedicated pump header. If you have a single-fan air cooler and your build is simple, leave CPU_OPT unpopulated, there are no negative consequences. The header will sit empty and your system will run fine.

Should I plug my AIO pump into CPU OPT or CPU FAN?

Plug the pump into CPU_FAN and the radiator fans into CPU_OPT or SYS_FAN. This satisfies the CPU_FAN boot-check requirement. If your motherboard has a dedicated AIO_PUMP or PUMP header, which runs at 100% speed continuously, use that for the pump instead and plug one radiator fan into CPU_FAN to satisfy the boot check. Never leave CPU_FAN empty with an AIO pump plugged only into CPU_OPT.

Can I plug a case fan into CPU OPT?

You can, but it is not recommended for typical case fan positions (front intake, rear exhaust, top exhaust). A fan plugged into CPU_OPT tracks CPU temperature, not chassis or ambient temperature. This means it may stay slow during GPU-heavy workloads when your case is warm but your CPU is idle. Use SYS_FAN or CHA_FAN headers for case fans and assign them a chassis-temperature-based curve in BIOS. The exception is a fan directly cooling the CPU VRM area, where CPU-temperature tracking is actually appropriate.

What is CHA FAN on a motherboard?

CHA_FAN stands for Chassis Fan, it is ASUS’s naming convention for what MSI and Gigabyte call SYS_FAN (System Fan). Both refer to the same type of header: a fan output designed for case fans that uses chassis or ambient temperature as its control source rather than the CPU thermal sensor. These headers are not mandatory for booting, are located toward the edges and bottom of the motherboard, and should be used for any fan that manages airflow within the case rather than directly cooling the CPU.

The Bottom Line

CPU_FAN and CPU_OPT are electrically identical, same voltage, same PWM signal, same CPU temperature source. The only real difference is that CPU_FAN is mandatory for booting and CPU_OPT is not. Use CPU_FAN for your primary cooler fan or AIO pump, use CPU_OPT for a second fan or AIO radiator fans, and save your SYS_FAN and CHA_FAN headers for case fans with their own temperature curves. Get those three right and your cooling setup will be clean, controlled, and quiet, which is the whole point of understanding cpu opt in the first place.