I too have this problem. This is a common problem with Polo's
see
https://www.vwaudiforum.co.uk/forum//...ead.php?t=3343
I have posted here and other forums before but no one is expert enough to be able to conclusively say what a simple fix is.
The fix seems to be quite complex.
Basically, there are tow motorised flaps in the heater control. One that controls the temperature and one that controls the fresh air intake. Both these motors are pretty unreliable and tend to get jammed intermitenly. I have checked this on my car with VAG-COM diagnostics cable connected to my laptop and get the following fault codes:
01271 - Positioning Motor for Temperature Flap (V68)
30-10 - Open or Short to B+ - Intermittent
01596 - Motor for Outside/Recirculated Air Flap (V154)
30-00 - Open or Short to B+
The fix is to replace these motors (V68 and V154). They are not cheap but the real problem is getting to them. To do it properly, the whole dashboad has to be removed to get to the heater mechanism. Which requires removing the steering wheel (which requires removing the battery lead which disables the airbags).
VW *********** quoted me £390 + vat for parts and labour !!
I have learned to live with the problem by trying one of these workarounds:
1. turn temp dial to coldest and close the central airvents for 1 minute or two while the vent dial is set to DEMIST. Then open the central vent fans -- that then lowers teh temp to the coldest.
or
2. turnthe fan speed to ZERO for a minute or two and then to 1. That will then reset the temp to the coldest.
Try that to see if it works for you.