Hey guys!
So I have a 2006 A3, and I noticed that my car has been overheatting. When I get out of my car i can feel the heat. About 2 weeks ago the radiators fans have started to turn on after I turn my car off so I decided to look into it. Then recently my dashboard gave a warning that I needed coolant and the gauge needle went from 1/2 to HOT. I felt a couple hoses around the car and they were very hot and some were cold(not warm.) So when I got home did some research before I messed with anything under the hood. I read through forums, and saw that it might be the thermostat or water pump, or head gasket. I was convinced that it was the thermostat so I recently replaced it and did a coolant change, but it seems to still heat up quickly and the fans are still turning on! Is there something that I'm missing, or something? Any help is appreciated, thank you!

 

Whenever I work on cooling related issues or maintenance there are 3 things I replace regardless of cause:

1. Waterpump
2. Thermostat
3. Cooling sensor(s)

The reason for all 3 is simple each one has something to do with the operation of the other part. So if only 1 of the 3 fails it will cause the other 2 to act up as well and deliver underwhelming performance. Usually if you do the work yourself adding just the other 2 parts that could potentially be the cause won't break the bank.
Most of the time it will be the thermostat like you mentioned however bad waterpump will have low or no flow causing the same symptoms but no error code (except overheating).

Another important step I've come across once with my Audi's:
Flush the cooling system. Not just dump it but flush it. I've had something gunk up inside my radiator once that I was able to flush out forcefully, so something to consider or check while you are at it - make sure there is good flow.