Dunno if this is a fault or not, but it seems pretty daft to me. The aircon on my wife's Mondeo (W reg 1.8TD) is turned on by pushing the fan speed dial, then a green light in the centre of the dial comes on. And stays on forever. ISTR that when we first got the car, pressing the dial again switched it off, but I wasn't paying that much attention at the time. The only thing that does turn it off, is turning the vent controls to '0', but it comes back on as soon as you switch to another setting.
Any simple solutions? (ICBA with complicated ones.)