After changing hard drive the fan runs continuously

The fan control software was a bad idea. That stuff can cause permanent damage to a system. I suggest you never go near that "technician" again. Overriding engineering is just plain dumb.

Try an SMC reset. If that does not help, take it to an Apple store for diagnosis.

Barry

edited

Your iMac's hard disk drive contains a temperature sensor with appropriate firmware. Replacing it with a generic hard disk drive without a sensor will cause the system to assume it's overheating, causing the fans to run at maximum speed all the time.

What can I do?

Replace it with the correct hard disk drive.

I fell into this trap a while back. You have to replace the hard drive with the same brand hard drive as you took out. So if it was a western digital drive that died, you need to replace it with a western digital drive. Turning down fan speed is never a good idea, when fans kick on it is usually for a good reason.

The other thing that can happen is that, during reassembly, a service technician fails to properly reconnect one of more of the various sensors he had to disconnect in taking the computer apart. As others have stated, the safety net is that the system is programmed to turn on the fans at full speed if it gets no data from a sensor. That means noise but the fans running in this safety net mode mean the computer doesn't fry.

Good point Allan, check out this repair guide https://www.ifixit.com/Guide/iMac+Intel+21.5-Inch+EMC+2308+Hard+Drive+Replacemen t/1766

Just be extremely carefully because the wiring is pretty small and fragile, but make sure all the correct connections are in there place. (When I did it, i used a small flat head screwdriver to help guide the cables in place.