So I found out today via vcds my 56 diesel is fitted with an auxiliary heater, albeit had fault codes. So naturally I wanted to find out more about it and if I could get it working. The Haynes manual provided me some info on its fuses and location, which is inside the dash behind the ac control unit and can accessed by removing glovebox and center console. I haven't found any info that tells me what conditions by when it starts to operate in the A3, I did find some info on golfs and i wanted to know if the following also applies to the A3's: "MK5 Golf and Jetta Diesel's have them. they are called a PET heater and it's bascially a 1KW heater positioned just upstream of the heater matrix. It only works under certain circumstances though. The heat setting has to be at least 90% of maximum, and re-circulation and ECON must be OFF. Here's a excerpt from a PDF file I have that is the MK5 self study program: An electrical auxiliary air heater is installed in diesel vehicles. The electrical auxiliary air heater is a combination of a PTC heater element with an integrated control unit. It is installed downstream of the heat exchanger and provides additional heating of the vehicle interior after cold-starting the egine. The electrical auxiliary air heater directly heats the air which enters the vehicle interior. It acquires all information needed for operation via the convenience CAN data bus. Activation conditions: The electrical auxiliary air heater is activated: - in the case of the 2C-Climatronic and Climatic: automatically via the CAN data bus; in the case of the heating system: if the occupants set the heating output to over 90% at the operating unit (analog signal),- if the water temperature is below 75°C, - if the engine speed is higher than 500 rpm, - if no load management system is active and - if the ECON button is not pressed."