Question: I have 2 RPi-4.

I've been using a fan inside a case that is designed by Canakit to be used with RPi-4. I've been using the fan for a few weeks by connecting it to constant 5V and ground. Everything was fine.

I am now trying to plug the fan on pin #3 so i can start it only when the cpu temp goes above a certain value.
It works well on my first RPi4 but not my second RPi4. On that one, i measured with a voltmeter and saw that voltage is effectively triggered on or off on that pin when the fan is not conencted... but when i connect the fan between ground and pin #3, it stays on for a fraction of a second and then turns off. This behavior only happens on my second RPi.

I did read 1 or 2 other questions very similar to this one and i understand that RPi would try to turn off the pin if too much current was drawn into it, but the current should not be too high, as it was working on the 5V pin. It also woks on my first RPi4. Any suggestion?

Thanks,

Joe

Answer: 
If you want to control a fan from GPIO you need some external circuitry, although this is minimal.

I use the following with my Pi4
[Fan control](https://raspberrypi.stackexchange.com/a/105820/8697)
A Raspberry should not be able to run any fan without some additional circuitry: GPIOs can supply 16 mA @ 3.3V, that is, it can supply only 52 mW. This is enough to light a led but not enough to drive a fan. I do not know any fan that could run with such small power.

You say that the fan worked in a Pi4 but not on a second one. Could you give some details of the fan and how it was connected to the first Pi4 ? Normally you should use either a relay or a transistor, as suggested by [Milliways](https://raspberrypi.stackexchange.com/users/8697/milliways). Knowing the details of the fan should allow a precise answer to your question.