Hello,

Last week I had the red coolant light start flashing on my way home from work. Pulled into a gas station for some coolant, refilled the tank and off I went. I understand I probably didn't get the correct coolant for Audis, but its better than nothing and will be replaced. I also discovered the small hose running from the expansion tank to the heater piping was leaking, so I replaced it.
Today, the light started flashing again. It started a few minutes after starting the car, turned off and turned on again as I got close to my destination. It did this on two separate trips. I have not seen any more leaks since replacing the hose and the expansion tank has plenty of coolant.
I am wondering if:
1. Air is in the system and needs to be bled out. (I imagine just run the engine with the pressure cap off and wait for the bubbles to stop.)
2. The level sensor is on it's way out. (could be unlikely since the PO replaced the expansion tank not long ago and they may have done the level sensor with it.)

 

The coolant warning will only go off if the coolant tank is low/out so you know the sensor is bad if there's plenty of liquid and the warning is still going off.

Air in the system definitely needs to be bled out if that's your issue. It could be that you just had a ton of air in there and the coolant is filling it back up, thereby emptying the tank and causing your light to go off - but that's only common after a coolant flush or drain, not driving normally.
It sounds like you still might have a leak somewhere. Do you see any puddles underneath? Can you pressure test it?

 

When I filled it last week the coolant was way low in the tank. Since then the coolant level remained steady in the tank, but I still got the red flashing light.
Today, however, I did a proper flush and refilled the system with Oreilly Universal Antifreeze. Started by filling the tank until the level stopped dropping, then I pulled the outlet heater hose, elevated the tank and saw coolant flowing from the heater hose. Put things back together and ran the car. Heat gauge climbed more than it had been and had steady heat from the vents.

To answer your questions; no puddles since last week and I will buy a pressure test kit if I have more problems in the future.

 

If the tank is indeed full, or at least near half, and the coolant warning light is going off, then yes it sounds like the sensor. You can also get that sensor warning if you forget to plug the sensor back into the bottom of the expansion tank which I often do when removing it to work on whatever underneath.
Since you did a flush, it may just be small air pockets finding their way out. So unless you really need to, I wouldn't spend money on a pressure kit just yet (you can also borrow them from bigger name parts stores like O'Reilly's and autozone.

Side note, audi is pretty strict about the type of coolant that goes into its cars. Something to do with the chemical compounds that help or hurt the seals, o-rings, etc. There's plenty of people who will chastise you for putting in non-eurpoean/audi fluid. Though some people will say just don't mix them and the generic stuff is fine, just doesn't have the best cooling properties and won't last quite as long. Take that as you may.

 

The funny thing is that I'll drive the car for a few minutes and then the light comes on. It will be on for a few more minutes then turn itself off. This makes me think it's got an intermittent bad sensor/poor connection.

I suppose I do need to bite the bullet for G13 coolant.

 

Is the sensor securely in its socket? It sounds like it might be moving around and losing connection with the electric pins it needs to stay in contact with.
Or is your coolant so low it's just sloshing around?