Econ button

i believe this indicates a fault in the aircon system.

Yup, I've had this. It's was sensor on the aircon radiator. Part is about £60 but the bumper needs to come off to get to it. VAGCOM will confirm if you have access to it. Search A4 forums for more details. Good news it doesn't breach the sealed system so you don't need the system regassed.

Probably the sensor, had mine done about 3 months ago. First time the new sensor (Generic) didn't fix the problem but the garage checked it over a couple of times again and finally fitted a second (OEM) sensor and only charged me for the difference in price between the two. No charge for the extra work involved.