I have a fault on my citroen xantia 1.9 TD SX with air con and climate control, when the heater is on full and the blower is on any position but lets say full on, the temp out of the vent is fairly warm, but within about 5 seconds it cools right down no matter what temp setting it is on it always cools right down BUT every now and then it stays a little warmer showing this is not a consistant fault. I have had the car conected to the citroen fault diagnostic machine but there was no fault shown other than the sensors in the vents and the ambient temp sensor showing way too high, so i changed the sensor but the fault is still there. I also removed the device that controlls the flap that allows heat into the vents thinking the vent was not opening fully, I manually altered the flap but this made no change. When I first encountered this problem I thought the heater matrix was blocked but I am not convinced of this as every time I flush out the matrix it is flowing fine and the water is usually quite clear.
Any help would be greatly appreciated Thanks Phil