Cant access fan control. Fan is constantly running.

Try resetting the SMC as follows.

1. Shut down your Mac, then unplug the power cable.
2. Wait 15 seconds.
3. Plug the power cord back in, then turn on your Mac.

An iMac controls fan speed automatically UNLESS you installed some third party app that claims the user can control fan speed manually or by some schedule. Bad idea if that’s what you did.