One of my regular jobs is servicing my brother bike, today I noticed an odd problem, it's a CBR125 (liquid cooled), and I realized I've never heard the radiator fan click on, so I started the bike and left it running to see if the fan would click on, not only did it not click on, but after a little while it started to idle oddly and when you revved it it would hang at 4K. 

I'm just gonna start measuring voltages and resistances in sensors and cleaning/normal checks. But does anyone know of any specific issues that can cause this? (Not CBR125 specifically, just in general with all bikes) 

(By the way, it's fuel injected, no carb, fucking EFI, pain in the ass on small bikes like this, seems a faf for such a little engine) 

Start easy.

Fan switches are notoriously unreliable. Bypass it to check the fan. 

The poor running may well be the temperatures going out of expected limits.

I second that