"Imac Fan won't turn off"

The Fan or Fans depending on the year model, always run at a quiet default speed of 1000 to 1200rpm.

If you have a fan that is running full blast, then first try resetting your iMac's SMC,

as per > How to reset the SMC of your Mac - Apple Support

If that doesn't help? Then you may have a faulty or disconnect temp. sensor causing the fan to run at full speed.

To check for a bad sensor, see > How to use Apple Diagnostics on your Mac - Apple Support

Since you replaced the hard drive in that iMac, if you did not 1) use a heat sensor cable, or 2) failed to connect the heat sensor cable to the replacement drive, then the iMac has no means to regulate its fan speed to the internal heat generated by the drive, and so it would run flat out in a state of panic…

copy and paste the following command in the Terminal followed by a return. It will prompt you for your administrator password, but not echo those entered characters as '*' in the Terminal:

sudo powermetrics -n 1 --samplers smc | grep "(CPU|GPU) die|Fan"



Run the Apple Diagnostic/Hardware Test.

You can also use a third party app called MacFanControl to check the fan speed and for a faulty or disconnected sensor. see > https://crystalidea.com/macs-fan-control

Just make sure that you understand how the Fan Speed Controls app works, before adjusting the fan speed. What ever you do, do not set the fan speed below the default or you will cook your iMac.