Hi Thanks for the update, very useful thread.
I have a Audi A3 2016 2.0 TDI and have a similar, I really could do with some advice please.
For the last 6 months or longer, I have had only a warmish air coming thru the heaters, I noticed when driving on the motorway, it does get a bit warmer, but most days it's cold.
I've also noticed there's coolant coming out of the side vessel of the expansion tank, I believe its spraying out due to the high pressure, also sometimes I can hear a gurgling/ steam noise when in the car. The coolant is very clear and the the engine oil has no cream.
I have recently taken the car to a VW specialist and they believe it's due to a clogged heater matrix, which I plan on changing myself this week.
Could the clogged heater matrix be causing the high pressure in the expansion tank?
Thanks