Planning the next
AO Silver Member
Join Date: May 2007
Location: United Kingdom
Does it behave the same whether the air is going to the footwell or the screen/face vents?
i.e. can you get warm air if the output is either to the footwell or to the windscreen.
If it is good one way, you may have a faulty mixed air sensor ( there are two, one for the footwell and one for the screen/air vents ).
Other than the mixed air sensors, I'd look at is the actuating motor on the side of the heater unit above the clutch pedal. This motor is supposed to feed back its position to the ecu. The motor operates a flap altering the proportion of air flowing through the heater matrix. If that motor isn't feeding its position back to the ecu properly it could be working in a digital fashion ( all hot or all cold ).