What he means is that the engine temperature sender may be faulty causing the ECU to either;

Go into a safety mode by firing up the fan all the time

or

The possibly faulty sender is fooling the ECU into thinking the engine is hot/cold when it is not.


Note that on the Focus there is not a coolant temp sender. There is a temperature sender that sits in a blind hole in the head that means the head (metal) temperature.

As above though, I don't think it likely to be an A/C issue but more likely to be the temp sender in the head.