Help Please Heating only on high

Sounds like a temperature flap motor has failed.

Edit:- this has been happening to all VW Group cars and probably many others since the beginning of using these flap motors, there is positional feed back built into them, and it is usual that carbon painted ring or its wiper that gets dirty so hands out duff info to the temperature controller.

One last gasp thing to try to see if it allows control of the temperature for a day/week/month, is:- demand Lo and wait for maybe 30 seconds, demand Hi and wait 30 seconds - repeat at least 10 times. What doing that is all about is, it should force the flap motor, and so its feedback wiper along the entire length of the track and then back again, sometimes that is enough to clean the carbon track up enough to allow the feedback to work as intended. The 30 seconds is just me guessing as I don't know how long these flap motors usually take to move from one end stop to the other and waiting longer means that you are achieving what you should be trying to and not just getting the flap motor to move a small way from where it is right now and back as that will not clean up anything much.