had the same problem and it turned out to be the temperature sensor on the wing mirror (easy to replace)
Graham24
June 2010
My C5 2005 2.0hdi has started to do the same today, suspect it may be water pump (just about spins on high revs to circulate the hot water), as when I rev the car warm air comes through, also i think it may be a dicky thermostat.
Secondly maybe the internal temprature sensor not working, located near the glovebox.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.