Fan running at 4649 RPM constantly I have an iMac from 2014. I have recently installed the Mojave update for macOS but the fan is turning at 4649 RPM all the time. I have installed a fan control app but the fan refuses to slow down.

Any ideas?

Thanks.
There are few reason why the Fan run at high speed.

1- is when something is actually running hot and Mac needs to cool it

2- a Fan controller SMC is misbehaving and needs rest

3- a hardware failure from a Thermal sensor

---------------Fixing

A 3D party Fan speed software will not fix anything 

1- to check that look in the Activity Monitor to see who is consuming CPU and shut it down.

2 - Reset your SMC, it is your model dependent so look it up. Since it can be tricky to do, watch the magsafe light briefly changing the color as you do the reset.

3 - if none of the above worked you might have a faulty temperature sensor. see Apple service for that.