Good morning 

I have an Audi A3 1.6 TDI and it’s started blowing cold air instead of warm/hot when switched on the warm settings , any ideas for a quick fix as it’s getting a bit cold in the mornings not to have a heater  

 

Sounds daft but have you tried the auto button and just set your temperature  ifound mine was just blowing cold but it can be a bit vague,it did work for me cheers

I'd recommend a diagnostic check to see if any fault codes flag up regarding the heater motor and controls etc. 

Also does the engine get up to temperature and stay constant? I ask as a failing thermostat could cause issues.