Sorry...only works on '97-'00 cars.
But for others, you can access the the diagnostics by holding the recirc and Air ↑ buttons for about 5 sec. Now you're in the diagnostic mode. The + - buttons allow you to scroll through the 36 different codes. The center button switches the display (left) from the code # and the actual value. To exit diagnostic mode, press Auto.
0c - ERL
1c - Oil temp
2c - Cabin temp as measured from the sensor mounted at the side of the dash.
3c - Outside temp - from AC air inlet
4c - Outside temp from ambient temp sensor on front grill (continuous)
5c - Outside temp feed from OBC display (5 sec. sampling and hold when stationary)
6c - Coolant temp
7c - Footwell discharge temp
8c - Sun sensor
9c - Sun sensor
10c - Cabin fan speed
11c - Cabin fan voltage
12c - Temp mix flap - 1=Cold, 100 = Hot
13c - Temp mix flap position
14c - Central flap cmmnd.
15c - Central flap position
16c - Footwell/defrost flap cmmnd.
17c - footwell/defrost position
18c - Recirc valve cmmnd - 1=Off, 100 = On
19c - Recirc valve position
20c - Vehicle speed in KPH, 1 sec. sampling, actual speed - no safety margin
21c - Engine RPM (hundreds) - 1 sec. sampling
22c - ??
23c - ??
24c - Display panel illum. voltage
25c - ??
26c - ??
27c - ??
28c - Fan speed
29c - ??
30c - Engine run time in sec. (255 max.)
31c - timing counter
32c - Display test
33c - Sftwr. ver.
34c - ??
35c - Outside temp from inlet sensor post filter
36c - Temp.
This unit is built by audi and the codes marked ?? have to do with different applications.