What he ^^ said, although to add make sure your heater position switch is not aimed at WINDSCREEN as the A/C is on then REGARDLESS, this is why it's coloured orange but not many know this.
If the fans are on all the time this indicates a major fault has been detected unless of course it is overheating for whatever reason.
It is indeed the CHT that indirectly switches the fans, this informs the ECU of the temperature of the metal of the cylinder head directly, this info is then used to control the temp gauge and fans as well as running the engine as well.
IIRC the temp gauge is set by CAN BUS to only display half unless near overheating.
Do the fans come on when the ignition is first switched on?
If not then remove the connector of the CHT this will log a fault and bring the fans on, when re connected the fault will be no longer and the fans will turn off, this shows if the CHT is working ok.
Martin