You've just finished building your PC, but there's one empty 4-pin header labeled CPU_OPT staring back at you. Should you plug something into it, or leave it empty? If you're using an AIO cooler or a high-end air cooler with dual fans, this little header might be exactly what you need—or it might be a trap that leads to confusing fan behavior. I've lost count of how many builds I've seen where someone plugged a case fan into CPU_OPT and then wondered why it was screaming at 100% speed while their GPU was melting. Let's clear up the confusion once and for all.
This guide covers everything: what the CPU OPT header actually does, how it differs from CPU_FAN and AIO_PUMP, whether you should connect your pump or fans to it, and how to fix common issues when things go wrong.
What Is the CPU_OPT Header and Why Does Your Motherboard Have It?
The CPU_OPT header—short for "CPU Optional"—is a secondary fan or pump connector found on most modern motherboards. It sits right next to the primary CPU_FAN header, usually within a centimeter or two of the CPU socket. Its job is simple: give you a second dedicated connection point for cooling hardware that's directly related to CPU thermal management.
The Official Purpose: A Secondary CPU Fan or Pump Connector
Here's the deal. High-end air coolers like the Noctua NH-D15 or the be quiet! Dark Rock Pro 5 ship with two fans. You could use a splitter cable to run both fans off the single CPU_FAN header, but motherboard manufacturers realized that's clunky. So they added CPU_OPT as a dedicated second header that mirrors the CPU_FAN signal.
When you plug a fan into CPU_OPT, it receives the same PWM control signal as the fan on CPU_FAN. Both fans spin in lockstep, responding to the same temperature source—the CPU's internal sensor. This is perfect for dual-fan tower coolers where you want both fans to behave identically.
For AIO coolers, CPU_OPT serves a different role. Many all-in-one liquid coolers have a pump that needs power, and CPU_OPT can handle that duty. The header typically provides the same 12V power and PWM signal as CPU_FAN, so your pump will run at a speed determined by your BIOS fan curve.
One thing to note: CPU_OPT is almost always a 4-pin PWM header, but some budget boards only support 3-pin voltage control on this header. Check your motherboard manual before assuming PWM works. I've seen builders plug a 4-pin PWM fan into a header that only supports DC control, and the fan ends up running at full speed constantly—not harmful, but definitely not ideal.
[Typical Motherboard Layout]
┌─────────────────────────────────────┐
│ CPU Socket │
│ ┌───────┐ │
│ │ CPU │ CPU_FAN CPU_OPT │
│ └───────┘ ● ● │
│ │
│ DIMM Slots │
│ ┌────┐ ┌────┐ │
│ │ │ │ │ AIO_PUMP │
│ └────┘ └────┘ ● │
│ │
│ SYS_FAN1 SYS_FAN2 SYS_FAN3 │
│ ● ● ● │
└─────────────────────────────────────┘
CPU_OPT vs. AIO_PUMP: What's the Real Difference?
This is where things get interesting. The AIO_PUMP header (sometimes labeled just "PUMP" or "W_PUMP") is designed specifically for water cooling pumps. Its default behavior is to run at 100% speed—always. No fan curve, no temperature-based adjustment. Just full power, all the time.
CPU_OPT, on the other hand, follows the CPU fan curve. If your CPU is cool, the pump connected to CPU_OPT will slow down. If your CPU heats up, the pump speeds up. For most AIO coolers, this is actually fine—pumps don't need to run at 100% constantly, and variable speed can reduce noise and extend pump life.
But here's the catch: some pumps require a minimum voltage to start. If your fan curve is set too aggressive (e.g., 20% speed at low temps), the pump might not receive enough power to spin up. This is why many motherboard manufacturers include a "Pump" or "Water Pump" preset in BIOS that sets a flat 100% curve.
| Header | Default Behavior | BIOS Options | Typical Use |
|---|---|---|---|
| CPU_FAN | Follows CPU temp fan curve | Full range: DC/PWM, custom curves | Primary CPU cooler fan |
| CPU_OPT | Mirrors CPU_FAN signal | Full range: DC/PWM, custom curves | Secondary CPU fan, AIO pump |
| AIO_PUMP | 100% speed | Often limited to full speed or fixed % | AIO pump, custom loop pump |
| Some motherboards—particularly higher-end ASUS and MSI boards—let you reconfigure AIO_PUMP in BIOS to act like a normal fan header. If you're using a custom water cooling loop with a separate pump controller, this can be useful. But for most builders, the safest bet is to connect your AIO pump to CPU_OPT and set it to 100% in BIOS. That way, you get pump speed monitoring (which AIO_PUMP also provides) without sacrificing the ability to adjust behavior later. |
CPU_OPT vs CPU_FAN: Key Differences You Need to Know Before Wiring
If you're staring at your motherboard and wondering which header gets which cable, here's the short version: CPU_FAN is mandatory, CPU_OPT is optional. But there's more nuance than that.
Signal Control and Temperature Source: The Critical Distinction
Both CPU_FAN and CPU_OPT are controlled by the CPU's internal temperature sensor—not the motherboard's system sensor. This is the single most important thing to understand about these headers.
Your motherboard has multiple temperature sensors. One lives inside the CPU die itself. Others are scattered across the board—near the VRM, near the chipset, near the M.2 slots. The CPU_FAN and CPU_OPT headers use the CPU sensor exclusively. The SYS_FAN and CHA_FAN headers use the motherboard sensors.
Why does this matter? Imagine you're gaming and your GPU is dumping heat into your case. Your case fans should spin faster to exhaust that hot air. But if those case fans are plugged into CPU_OPT, they'll only respond to CPU temperature—which might be perfectly cool because your CPU isn't under heavy load. Result: your case turns into an oven while your CPU fan idles.
Temperature Sensor Control Map:
┌─────────────────────────────────────────────┐
│ CPU Temperature Sensor │
│ ├── CPU_FAN header │
│ ├── CPU_OPT header │
│ └── (Sometimes) AIO_PUMP header │
│ │
│ Motherboard Temperature Sensor │
│ ├── SYS_FAN1/2/3 headers │
│ ├── CHA_FAN headers │
│ └── (Sometimes) AIO_PUMP header │
└─────────────────────────────────────────────┘
Another critical difference: most motherboards refuse to boot if nothing is connected to CPU_FAN. This is a safety feature—the board assumes no CPU cooling means imminent damage. CPU_OPT has no such failsafe. You can leave it empty and your system will boot normally.
Can You Plug a Case Fan into the CPU_OPT Header?
Yes, it's electrically safe. The header provides the same 12V power and ground as any other fan header. I've done it myself when I ran out of SYS_FAN headers on a budget board. Nothing exploded, no magic smoke escaped.
But here's the "should you" part: no, not for a permanent solution. As I mentioned above, the case fan will spin based on CPU temperature, not system temperature. During a GPU-heavy workload, your case fans might stay at minimum speed while your GPU cooks. During a CPU stress test, they'll spin up to maximum even if your case is ice cold.
If you're in a pinch and need to get a system running temporarily, go ahead and use CPU_OPT for a case fan. It won't damage anything. But invest in a fan splitter or a powered fan hub for a permanent fix. A simple splitter cable costs less than $10 and lets you run two case fans off a single SYS_FAN header.
How to Use the CPU_OPT Header for Water Cooling and AIO Pumps
If you're building with an AIO cooler, the CPU_OPT header is your friend. Here's how to wire it up correctly.
Step-by-Step: Connecting Your AIO Pump to CPU_OPT
Step 1: Locate the CPU_OPT header. It's almost always right next to CPU_FAN, near the top of the motherboard. Check your manual if you can't find it—some boards place it slightly further away, especially on compact ITX designs.
Step 2: Connect the pump cable. Most AIO pumps have a 3-pin or 4-pin cable coming from the pump block. Plug it into CPU_OPT. The connector is keyed, so you can't insert it backwards.
Step 3: Enter BIOS. Restart your system and press the BIOS key (usually Delete, F2, or F12 depending on your motherboard brand). Navigate to the fan control section—this is called Q-Fan Control on Gigabyte, Fan Xpert on ASUS, and Smart Fan Mode on MSI.
Step 4: Set CPU_OPT to "Full Speed" or "PWM Mode." For a pump, you want it running at 100% consistently. Some boards have a dedicated "Pump" preset that does exactly this. If your board only offers manual fan curves, set a flat curve at 100%.
Step 5: Verify the pump RPM. After saving and exiting BIOS, check your monitoring software (HWInfo, your motherboard's utility, or BIOS itself) to confirm the pump is spinning and reporting RPM. If you see 0 RPM, something's wrong—check the connection.
Best BIOS Settings for CPU_OPT with an AIO Cooler
For AIO pumps, I recommend a flat 100% fan curve. Here's why: pumps are designed to run at full speed. They're not like fans where lower speed means less noise and acceptable cooling. A pump running at 50% speed might not circulate coolant fast enough, leading to higher CPU temperatures and potential pump damage over time.
If you're using a dual-fan air cooler on CPU_OPT, you'll want a different approach. Set both CPU_FAN and CPU_OPT to the same custom fan curve. This ensures both fans spin at the same speed, maintaining balanced airflow through the heatsink.
Most modern motherboards have presets that make this easy:
- ASUS: In Fan Xpert, select "Pump" for CPU_OPT if you're using an AIO. For air coolers, select "Standard" or "Silent" and apply the same curve to both headers.
- MSI: In Smart Fan Mode, choose "Pump" for AIO setups. For dual-fan air coolers, set both headers to "Custom" and copy the same curve.
- Gigabyte: In Q-Fan Control, select "Full Speed" for pump headers. For air coolers, use "Auto" and ensure both headers have identical curves.
One tip I've learned from years of building: if your AIO pump has a separate RGB cable, don't plug it into CPU_OPT. That's a lighting connection, not a power connection. It belongs on an RGB header or hub.
CPU_OPT Header Not Working? A Step-by-Step Troubleshooting Guide
Sometimes things go wrong. Here's how to diagnose and fix common CPU_OPT issues.
Common Symptoms: No RPM Reading, Fan Not Spinning, or Full Speed Always
Symptom 1: Fan or pump not spinning at all. First, check the physical connection. Is the cable fully seated? Is it plugged into the right header? I've seen builders accidentally plug a pump into a SYS_FAN header and wonder why it's not spinning—the pump needs the higher current capacity of a CPU or AIO header.
Next, check BIOS. Some motherboards allow you to disable unused headers. If CPU_OPT is disabled, nothing plugged into it will work. Enable it and try again.
Symptom 2: RPM reading is 0 in BIOS. This usually means the header isn't detecting the fan or pump. If you're using a 3-pin fan on a 4-pin header, the RPM signal should still work—the third pin carries the tachometer signal. If you're using a 4-pin PWM fan, make sure it's actually a PWM fan and not a 3-pin fan with a 4-pin connector (some cheap fans do this).
Symptom 3: Fan runs at 100% always. This is almost always a PWM vs. DC mode mismatch. If your fan is 3-pin and the header is set to PWM mode, the fan will run at full speed because it can't understand the PWM signal. Switch the header to DC mode in BIOS. Conversely, if you have a 4-pin PWM fan and the header is set to DC mode, the fan will also run at full speed.
| Symptom | Possible Causes | Fixes |
|---|---|---|
| No spinning | Loose connection, disabled header, faulty fan | Reseat cable, enable header in BIOS, test with different fan |
| 0 RPM reading | 3-pin fan on PWM header, faulty fan, damaged header | Switch to DC mode, test with known-good fan, check header pins |
| Always 100% speed | PWM/DC mismatch, BIOS curve set to 100% | Change header mode in BIOS, adjust fan curve |
BIOS Settings and Hardware Checks to Resolve the Issue
Start with the simplest fix: update your motherboard BIOS. I've seen firmware bugs cause fan headers to behave erratically—one Gigabyte board I worked with had a BIOS version that ignored CPU_OPT entirely until updated.
If updating doesn't help, try a different fan or pump on the CPU_OPT header. This rules out a faulty device. If the new device works, the problem is your original fan or pump. If it doesn't work, the header itself might be damaged.
For physically damaged headers, you have two options. First, use a fan splitter from CPU_FAN to power both devices. Second, use a powered fan hub that draws power from the PSU and only uses the motherboard header for the control signal. Both work fine, though a powered hub is better for multiple fans since it doesn't overload the header.
CPU_OPT Header Specs and Compatibility: Amps, Watts, and 3-Pin Fans
Before you plug anything into CPU_OPT, you should know its electrical limits.
Electrical Limits: How Much Power Can CPU_OPT Handle?
Most CPU_OPT headers are rated for 1A at 12V, which equals 12 watts. Some higher-end boards support 2A (24W), but 1A is the industry standard. Check your motherboard manual for the exact spec—it's usually listed in the fan header section.
Exceeding the header's current limit can damage the header, the fan, or the motherboard itself. If you're connecting multiple fans via a splitter, add up their current draw. A typical case fan draws 0.2-0.3A. Two fans on a splitter is fine. Three or more might push the limit.
For multiple fans, use a powered fan hub. These devices draw power directly from the PSU via a SATA or Molex connector, and only use the motherboard header for the PWM control signal. This is the safe way to run 4+ fans from a single header.
| Motherboard Brand | Typical CPU_OPT Spec | Notes |
|---|---|---|
| ASUS | 1A / 12W | Check manual for exact spec |
| MSI | 1A / 12W | Some boards label it PUMP_FAN1 |
| Gigabyte | 1A / 12W | Often labeled CPU_OPT/PUMP |
| ASRock | 1A / 12W | Sometimes labeled CPU_FAN2 |
Does CPU_OPT Support 3-Pin Fans? (Voltage vs. PWM Control)
Yes, most CPU_OPT headers support both 3-pin (DC) and 4-pin (PWM) fans. The header has four physical pins, but the fourth pin (PWM control) is only used by 4-pin fans. A 3-pin fan will still receive power and ground, and the tachometer signal will work.
The key is setting the correct mode in BIOS. If you're using a 3-pin fan, set the header to DC mode. This controls fan speed by varying the voltage (12V for full speed, lower voltages for slower speeds). If you're using a 4-pin PWM fan, set the header to PWM mode. This keeps voltage constant at 12V and controls speed via the PWM signal.
Here's the gotcha: if you set the header to PWM mode and plug in a 3-pin fan, the fan will run at full speed. The fan has no PWM controller, so it ignores the PWM signal and just runs at 12V. This won't damage anything, but it's noisy and inefficient.
Brand-Specific CPU_OPT Headers: ASUS, MSI, Gigabyte, and ASRock
Different motherboard brands handle CPU_OPT slightly differently. Here's what you need to know.
Where to Find CPU_OPT on Popular Motherboard Brands
ASUS: CPU_OPT is almost always located directly above or to the right of the CPU socket, right next to CPU_FAN. On ROG boards, it's clearly labeled with white text. ASUS also includes a dedicated AIO_PUMP header on most boards, so you have three options for CPU cooling connections.
MSI: MSI labels this header CPU_OPT on most boards, but some models call it PUMP_FAN1. It's typically located near the CPU socket, often between CPU_FAN and the DIMM slots. MSI's BIOS makes it easy to switch between fan and pump modes.
Gigabyte: Gigabyte often labels this header CPU_OPT or CPU_OPT/PUMP. On some boards, it shares control with CPU_FAN—changing one affects the other. This is fine for dual-fan air coolers but can be confusing if you're trying to set different curves for fans and pumps.
ASRock: ASRock typically labels this header CPU_OPT or CPU_FAN2. Location varies more than other brands—check your manual. Some budget ASRock boards only have a 3-pin CPU_OPT header, so PWM fans won't work at variable speed.
| Brand | Typical Label | Location | Notes |
|---|---|---|---|
| ASUS | CPU_OPT | Next to CPU_FAN | Dedicated AIO_PUMP header on most boards |
| MSI | CPU_OPT or PUMP_FAN1 | Near CPU socket | Easy mode switching in BIOS |
| Gigabyte | CPU_OPT or CPU_OPT/PUMP | Next to CPU_FAN | May share control with CPU_FAN |
| ASRock | CPU_OPT or CPU_FAN2 | Varies by model | Check manual for exact location |
Pinout Differences and Compatibility Notes
The standard 4-pin fan header pinout is universal: Pin 1 is Ground, Pin 2 is +12V, Pin 3 is the tachometer (RPM signal), and Pin 4 is the PWM control signal. CPU_OPT follows this standard on virtually all motherboards.
Standard 4-Pin Fan Header Pinout:
┌─────────────────────────┐
│ Pin 1: Ground │
│ Pin 2: +12V Power │
│ Pin 3: Tachometer (RPM) │
│ Pin 4: PWM Control │
└─────────────────────────┘
One warning: don't use proprietary cables from AIO coolers or fan hubs that might have different pinouts. Some older Corsair and NZXT devices used non-standard connectors. Always use the cables that came with your hardware, or standard fan extension cables.
FAQ
Can I plug a case fan into the CPU_OPT header?
Yes, it's electrically safe and won't damage anything. However, it's not recommended for permanent use because the header is controlled by the CPU temperature sensor, not the motherboard sensor. Your case fan will spin based on CPU heat, which means it might run too fast during CPU-heavy tasks and too slow during GPU-heavy tasks. Use a fan splitter or hub to connect case fans to SYS_FAN headers instead.
What is the difference between CPU_FAN and CPU_OPT?
CPU_FAN is the primary header required for booting—most motherboards won't start if nothing is connected to it. CPU_OPT is a secondary header that mirrors CPU_FAN's signal. Both use the CPU temperature sensor for speed control. CPU_OPT is ideal for a second CPU fan on a dual-fan air cooler or for an AIO pump.
Should I connect my AIO pump to CPU_OPT or CPU_FAN?
Either works, but CPU_OPT is often preferred because it leaves CPU_FAN dedicated to the radiator fans. This gives you separate control over pump speed and fan speed. Some pumps require a full-speed header, which you can configure in BIOS for CPU_OPT. If your motherboard has a dedicated AIO_PUMP header, that's the best choice.
Why is my CPU_OPT header not detected in BIOS?
First, check if the header is disabled in BIOS—some boards allow you to turn off unused headers. Ensure the fan or pump is properly connected. Try updating your motherboard BIOS to fix potential firmware bugs. If the issue persists, test with a different device to rule out a hardware fault. If the header is physically damaged, use a fan splitter from CPU_FAN or a powered hub as a workaround.
Conclusion
The CPU OPT header is one of those motherboard features that seems mysterious until you understand its purpose. It's a secondary connection point for CPU cooling hardware—whether that's a second fan on a massive air cooler or the pump on your AIO. It mirrors the CPU_FAN signal, responds to CPU temperature, and gives you flexibility in how you wire your cooling system.
The key takeaways: CPU_FAN is mandatory, CPU_OPT is optional. Use CPU_OPT for secondary CPU fans or AIO pumps, not for case fans. Check your motherboard manual for specific specs and BIOS options. And if something's not working, start with the simplest fixes—check connections, verify BIOS settings, and update your firmware.
Have you used your CPU_OPT header for a pump or fan? Share your setup in the comments below, or ask a question if you're still unsure about your specific motherboard model!