The fans on my iMac are running full speed constantly. How do I fix this?

Reset the SMC as per > Reset the SMC of your Mac - Apple Support

Desktop computer

1. Shut down your Mac, then unplug the power cord.
2. Wait 15 seconds, then plug the power cord back in.
3. Wait 5 seconds, then press the power button to turn on your Mac.

If you have installed a third party Fan Control Utility on the iMac?

Consult the developers site for the proper use or uninstall it and reset the SMC.

If you have opened the iMac to replaced the drive or did any other DIY work?

Then you may have damaged or unhooked a temp' sensor that is responsible for controlling the fan speed.

As nothing has helped so far...

iMacs have been in continuous production for 26 years and we to not know which you have. Some have no fans and some have up to three. If you tell us the sub-model (the bit with a year), we many be able to better assess if this is a hardware problem or something else. The sub-model also make a difference in how to read diagnostic codes that can reveal fan hardware issues.

Example: Apple Hardware Test (older Macs) and Apple Diagnostics (newer Macs) can show a different code for a failed fan and a failed fan sensor. The latter is much harder to fix, usually requiring a new logic board.

For Macs running macOS 10.7 "Lion" and newer, doing "About this Mac.." from your Apple menu will show your iMac's SUB-MODEL, the key to further and better diagnosis in this setting where we can neither seen nor touch your computer. Example:

For security reasons, please DO NOT post the serial number.

Thanks!

Have you blown the dust out of the intakes lately?

What temperatures does this App report? Especially the GPU.

This App can be used to speed up the fans, all my iMacs would be dead without it.

Never ever ever drop fan speeds below the speed that the Mac itself is calling for.

MacFansControl…

https://www.crystalidea.com/macs-fan-control

OK good, thanks for the update. Now reinstall the MacFanControl utility that BDAuqa suggested and look for a bad or negative Temperature Sensor Value.

Sorry I was editing when you replied.

If you have installed a third party Fan Control Utility on the iMac?

Consult the developers site for the proper use or uninstall it and reset the SMC.

If you have opened the iMac to replaced the drive or did any other DIY work?

Then you may have damaged or unhooked a temp' sensor that is responsible for controlling the fan speed.