I got my car back from the mechanic on Saturday. Drove it around a few miles. Everything seemed fine. On Sunday went to start it up I get temperature warning light. I discovered that the coolant reservoir was empty. There was no sign of leaking underneath the car.

So, I went to Pep Boys and bought some coolant (yellow). I started pouring it into the reservoir, but it started overflowing. Why? The reservoir tank seems to be blocked or clogged with something. Am I missing something obvious?

How bad is it that I'm using yellow coolant instead of the Audi recommended G12?

Appreciate your wisdom.