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

CPU_FAN header controls the primary processor cooler and is required for booting, while CPU_OPT is a secondary optional header for an additional CPU-related fan or pump, running on the same PWM signal but with no boot dependency.

Last updated: May 2026

Quick Answer: CPU OPT vs CPU FAN

The short version: both headers are 4-pin PWM, both pull their fan curve from the CPU thermal sensor, and they output an identical signal. The only practical difference is that CPU_FAN is mandatory for boot (a missing fan triggers a POST error), while CPU_OPT is optional. Use CPU_FAN for your main cooler fan or AIO pump, and use CPU_OPT for a second fan in push-pull configurations, AIO radiator fans, or a custom loop pump.

What Does CPU OPT Mean?

CPU OPT stands for CPU Optional. The header is a secondary 4-pin PWM fan connector on your motherboard, electrically identical to CPU_FAN but explicitly designed as a supplemental output that won’t halt boot if it’s left empty. The “optional” in the name refers to its boot behavior, not to whether the header itself is useful, it’s perfectly active and controllable when something is plugged into it.

The CPU_OPT header outputs the same PWM signal as CPU_FAN. Both headers source their fan curve from the CPU’s internal thermal sensor (Tdie on AMD, Package on Intel), which means a fan on CPU_OPT will ramp up and slow down in lockstep with whatever is on CPU_FAN. The voltage is identical (+12V across pin 2), the speed is controlled the same way (PWM signal on pin 4), and the tach reading is monitored on pin 3.

If you’ve ever seen labels like CPU_FAN2, CPU_FAN_OPT, or just CPU_OPT1 on a motherboard PCB or in BIOS, they all refer to the same thing. ASUS, MSI, Gigabyte, and ASRock use slightly different naming conventions, but the function is identical across vendors.

CPU OPT Use Cases: When to Use the Optional Header

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.

AIO Liquid Cooler: Pump and Radiator Fans

For an AIO liquid cooler, the question of which header gets what depends on whether your board has a dedicated PUMP header.

• Board has AIO_PUMP / PUMP header: Plug the pump into the dedicated PUMP header (runs at 100% continuously, which is what a pump needs). Plug one radiator fan into CPU_FAN to satisfy the boot check. Remaining radiator fans go on CPU_OPT or a fan hub.
• Board has no PUMP header: Plug the pump into CPU_FAN. This satisfies the boot check, and the pump will run at its variable PWM curve, which is usually fine for modern AIOs that handle their own pump speed internally. Radiator fans go on CPU_OPT.

Never leave CPU_FAN empty with the pump plugged only into CPU_OPT. The system will throw a POST error on boot or refuse to start entirely.

Can I Use CPU OPT for a Case Fan?

Technically yes, the fan will spin. But it’s 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. 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 and assign them a chassis-temperature-based fan curve in BIOS.

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, Z790, X870E, and Z890 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.
• Y-splitter on CPU_FAN: A simple Y-cable splits the CPU_FAN signal across two physical fans. Both fans receive the identical PWM signal. The downside: the motherboard only sees one tach reading (typically from the primary fan), so you lose individual RPM monitoring on the second fan.

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.

What does CPU OPT mean on a motherboard?

On a motherboard, CPU OPT means “CPU Optional”, a supplemental fan header tied to the CPU thermal sensor. The header carries +12V power and a PWM signal identical to the primary CPU_FAN header, but the motherboard firmware treats it as non-essential. You can leave CPU_OPT empty without any boot issues, but when populated, it gives you a second CPU-linked fan output without needing splitters or fan hubs.

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.

What header should I use for AIO radiator fans?

AIO radiator fans should go on CPU_OPT as the cleanest option, since CPU_OPT tracks CPU temperature and will ramp the radiator fans up in sync with CPU load. If you have multiple radiator fans (240mm or 360mm AIOs typically have 2-3 fans), connect them via the AIO’s own fan splitter cable (usually included in the box) and plug the resulting single connector into CPU_OPT. Alternatively, plug them into SYS_FAN headers with a manual curve set to mirror CPU temperature behavior, but CPU_OPT is the cleaner solution if the header is available.

Can I use a Y-splitter on CPU_FAN instead of CPU_OPT?

Yes, a Y-splitter or fan splitter cable works on CPU_FAN and lets you run two fans from a single header. Both fans receive the identical PWM signal and will spin at the same speed. The trade-off is that the motherboard only reads tach (RPM) data from one fan, typically the one on the primary lead. You lose individual RPM monitoring for the second fan, but most BIOS environments handle this fine. CPU_OPT is the cleaner solution if your board has one, but a splitter on CPU_FAN is a perfectly functional alternative.

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.

What kind of cable does CPU OPT use?

CPU_OPT uses a standard 4-pin PWM fan connector, the same physical connector as CPU_FAN, SYS_FAN, and most other modern fan headers. The connector has a small plastic key on one side to prevent incorrect insertion. 3-pin DC fans will also fit and work on CPU_OPT (they connect to the first three pins), but they’re voltage-controlled rather than PWM-controlled, so their speed range and quietness will be more limited. Most modern CPU coolers ship with native 4-pin PWM fans.

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 header types right and your cooling setup will be clean, controlled, and quiet, which is the whole point.