Planning the next
AO Silver Member
Join Date: May 2007
Location: United Kingdom
On the phase 2 the heater, I think, is permanently in circuit. There's no valve as there is on phase 1s.
The way it shuts off the heat is by way of a motor controlled flap that mixes incoming cold air ( either from outside or via the aircon ) with air going through the heater matrix.
So two possibilites come to mind.
1. The motor unit controlling the mixing flap has either failed or become unconfigured.
2. The mixed air temperature sensor has failed.
The second option may be the easiest to test first as there are two sensors ( one for face/windscreen level and another for foot level ).
Whichever way the air is flowing at the moment ( foot level or face/screen level ), direct it the other way. If the problem goes away, the likelihood is that of mixed air sensor failure. This could probably be confirmed by swapping the position of the two sensors and seeing if the problem moves with the sensor.
If the situation persists I'd look at the actuating motor for the air mixing flap. The motor provides feedback in respect of its position to the ECU by way of a potentiometer attached to the motor.
It may also be worthwhile trying a climate control ECU reset - but I only know how to do that on a 156 at the moment.