SYS_FAN2 and SYS_FAN3 Explained | Generated by AI
Question: What do the SYS_FAN2 and SYS_FAN3 headers mean on this MSI motherboard?
Answer:
SYS_FAN2 and SYS_FAN3 are two of four 4-pin system/chassis fan headers (SYS_FAN1~4) on the board, used to power and control case fans — as opposed to CPU_FAN1 (CPU cooler) and PUMP_FAN1 (AIO pump).
Pinout (4-pin):
| Pin | PWM Mode | DC Mode |
|---|---|---|
| 1 | Ground | Ground |
| 2 | +12V | Voltage Control |
| 3 | Sense (RPM feedback) | Sense |
| 4 | Speed Control Signal | NC |
Electrical/mode specs:
| Connector | Default mode | Max current | Max power |
|---|---|---|---|
CPU_FAN1 |
PWM | 2A | 24W |
PUMP_FAN1 |
PWM | 3A | 36W |
SYS_FAN1~4 |
DC mode | 2A | 24W |
Key detail: SYS_FAN1~4 default to DC mode, not PWM. That means MSI controls their speed by varying the 12V rail voltage (pin 2), not via a PWM duty-cycle signal on pin 4. If you plug in a 4-pin PWM fan expecting pin-4 PWM control, it may just spin at a fixed/near-max speed until you flip it to PWM mode.
Where to change this: BIOS → Hardware Monitor panel, per-fan setting to switch between DC/PWM mode and enable/disable Smart Fan (temperature-curve) control. If a 3-pin DC-only fan is plugged into a header left in PWM mode (or vice versa), speed control won’t behave correctly — always match the fan’s actual signal type to the header mode in BIOS.
Physical location: on this board’s layout, SYS_FAN1 sits near the bottom edge (below JCI1), and SYS_FAN2/SYS_FAN3 are clustered near the right-side USB headers (JUSB1/JUSB2), with SYS_FAN4 up near the DIMM slots/ATX_PWR1 area — so which physical fan you cable to which header is mostly about cable routing/reach, not a functional difference (all four are equivalent DC-mode headers).
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