Symptoms
- Fan runs at full speed all the time regardless of temperature
- Fan control software or BIOS doesn't let you adjust that specific fan's speed
- Fan connector has 3 pins but the header has 4, or vice versa
- Fan behaves normally when moved to a different header
Likely causes
- Common3-pin (voltage-controlled) fan plugged into a 4-pin PWM header without proper handlingSome headers handle this fine automatically, but others default to full speed.
- Common4-pin PWM fan plugged into a 3-pin only headerWithout a PWM signal pin, the fan often just runs at full voltage/speed.
- PossibleBIOS fan header setting configured for the wrong control mode (DC vs PWM)Many boards let you manually choose DC or PWM mode per header, and a mismatch causes odd behavior.
What to do
- Check the physical connector: 3-pin fans have three wires, 4-pin PWM fans have four, with the extra wire carrying the speed control signal.
- Note which header type the fan is currently connected to, since most 4-pin headers do accept 3-pin connectors physically (offset by one pin) but not vice versa.
- In BIOS, look for that specific fan header's settings and check if it's set to DC or PWM mode, switching to match the fan type.
- If the fan is 3-pin and connected to a PWM header, try setting that header to DC/voltage mode in BIOS.
- If a 4-pin PWM fan is on a 3-pin-only header, it will typically just run at full speed since there's no PWM signal wire connected.
- Move the fan to a header type it's actually designed for whenever possible, rather than only adjusting software settings.
Tools and parts
- Motherboard manual
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.