Having a similar issue on my 2006 V8 petrol. Recently re-connected the heater matrix as it had been bypassed in the engine bay and after that I realise why it was disconnected...
One side of the matrix is always receiving hot water from the coolant circuit, the other side is cold. This is coming from the split control valve in the engine bay.
Once I plugged in my iiD tool I checked for what live information I could read from the HVAC system. It is saying that the internal car temperature is 40 degrees which it clearly isn't. I cant seem to get a value for the air temperature coming off the heater coils to see if these sensors are working correctly but due to this 40 degree figure I am thinking that the HVAC control panel is faulty as the internal sensor is behind that little grille on the front plate in the dash. I have had the whole thing apart and everything seems visually OK. Reading the workshop manual, this seems to control everything so if its playing up, it might be giving false information to the temperature control valve. Also my fan speed control has a mind of its own, which reinforces my diagnosis.
If you have access to an iiD tool then check out the live values for anything fishy. It does say this in the workshop manual about faults in the control unit.
Inputs and Outputs
Five electrical connectors provide the interface between the ATC module and the vehicle/heater assembly wiring.
Both the manual and automatic systems receive ambient temperature, engine coolant temperature, engine speed and vehicle speed inputs in K bus messages from the instrument cluster. If the K bus messages are missing or faulty, the ATC module adopts the following default values:
Ambient temperature = 0 °C (32 °F).
Engine coolant temperature = 80 °C (176 °F).
Engine speed = 800 rev/min.
Vehicle speed = zero.
If a fault develops in the input from the temperature selector switch on the control panel, the ATC module adopts a default
value of 24 °C (75 °F).
Hope that helps...
Ste
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