Regular fans for 3D printers have just two wires, power and ground, and we vary the average supply voltage to the fan using PWM to change its speed.
However 4-wire computer fans like Noctuas are different, these fans expect a steady 12V power supply and a separate 5V PWM signal to set their speed. Check the pinout of your fan’s connector here:
https://noctua.at/en/what-pin-configuration-do-noctua-products-use
Continue reading Driving a 4-pin computer PWM fan on the BTT Octopus using Klipper