What is the best way to diagnose Fan issues with a 21.5" iMac?

[jnzbunting]

Hi all,

My 21.5" iMac (14,1) has begun having some weird issues with the fan. Under idle use - literally nothing open, the fan will ramp up to c. 4200rpm for a second or two, then stop - this happens about every 30 seconds or so.

1. There are no rogue processes running, this is on a clean install of MacOS.
2. Temperatures are all reporting as normal - (30-40c) across most sensors.
3. Have completed multiple SMC resets - no change.
4. Have completed a diagnostic test - no errors found.
5. Have tried manually controlling the fans using both SMCFanControl and Macs Fan Control - both of which are unable to control the fans (any changes i make in both of those apps are having no effect and wont control the fan at all)
6. I've performed various MacOS clean installs - even restored the the original OS it shipped with - no change.
7. It's a stock system - no SSD or Ram upgrades.

Other than the fan issue - the system runs perfectly.

I'm beginning to run out of ideas. I know its an older machine, It needs to run one peice of CNC equipment so will have a very light workload, and will be a perfect machine to do so.

Thanks in advance!
Joe