My sister lives in France and has 2000 A6 2.5TDi.
She says that the aircon will only work when the recirc button is pressed. With the aircon running normally the ECON light is permanently on unless the recirc button is pressed. When the recirc is pressed then the compressor must be working as she says the air is cold.
I have advised her to check the pollen filter, but am not sure beyond that. Possibly a fault in the control unit.
Anyone have any ideas?