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With the ignition on (or the engine running) press the 'windscreen vent' and 'recirculation' buttons simultaneously for about 5 seconds
The left display (which normally shows climate control temperature) will switch to either a 'c-code' or its associated sensor reading.
If a sensor reading is showing, press the 'central dash vent' button to jump back to the 'c-code' selection
Use the '+ and -' buttons under the left display (normally used to set climate control temperature) to scroll through the 'c-codes' until you get to one you need
Press the 'central dash vent' button to display the associated value for the 'c-code'
Use the 'central dash vent' button to switch between the 'c-code' and value displays
Press the 'Auto' button to exit this hidden function and switch back to normal climate control functionality
The various 'c-codes' and their associated meanings are as follows (yes, the list is far from complete):
0c - ERL
1c - Oil Temp?
2c - Inside temp. Sensor mounted in the aspirator assembly at the side of the dash
3c - Outside temp. Sensor located inside the air inlet of the A/C unit * (see below)
4c - Outside temp. Sensor located in front grill of the radiator * (see below)
5c - Outside temp. (matches OBC outside temp display)
6c - Coolant temp.
7c - Footwell discharge temp.
8c - Sun sensor (dash top)
9c - Sun sensor
10c - Passenger compartment fan speed
11c - Passenger compartment fan voltage
12c - Temperature mix Flap command 1=COLD, 100=HOT
13c - Temperature mix Flap position
14c - Central Flap command
15c - Central Flap position
16c - Footwell/Defrost Flap command
17c - Footwell/Defrost Flap position
18c - Recirculation Valve command 1=OFF, 100=RECIRC
19c - Recirculation Valve position (feedback)
20c - Vehicle speed in kph, updating only once per second. (real speed, not speed+safety margin as in the speedometer)
21c - Engine RPM in hundreds. This too only updates once per second
22c - ?
23c - ?
24c - Sun sensor, exterior lights switch & panel lights control (term. 58 & 58d voltage) - used for A/C panel display illumination
25c - ?
26c - ?
27c - ?
28c - Fan speed?
29c - ?
30c - Engine run time in seconds (255 max.)(=0xff)
31c - Timing counter
32c - Displays test
33c - Software version?
34c - ?
35c - Outside temp. from inlet sensor (filtered?)
36c - temp?
* - The data is fed to the Climatronic from the instrument cluster. When not moving, the instrument cluster OBC temp display retains it's last setting until moving. This is to prevent heat emanating from the radiator affecting the temperature sensor. The A/C unit uses the lower of the two outside air temp values in determining fresh-air temp.