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.
Maz Aslam
December 2009
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