I found this info online - so I should be able to diag the issue.
This is a handy tip to allow you to use your AC display to access diagnostic info on your Boxster..
First, press and hold the Recirculating Button, then press both Temperature Buttons (+ -). To enter the Diagnostics, you hold down the Recirculating and Air Up buttons simultaneously for 5 sec. The + - will then scroll up/down through a list of 'C' codes. Pressing the Center Vent Button will alternate between the 'C' code with it's actual value. To exit and return to normal operation, press the Auto Button. Here is a list of the available codes:
0c - ERL
1c - Oil Temp
2c - Inside Temp - From the Sensor mounted in the Aspirator Assembly at the side of the Dash 3c - Outside Temp - From the Sensor inside the Air Inlet of the A/C Unit 4c - Outside Temp - From the Sensor in Front Grill of the Radiator. 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 Temp Sensor. The Climatronic uses the lower of the two values in determining Fresh-Air Temp.
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, updates once per second. This is True Speed, not the Speed+Safety Margin displayed by the Speedometer.
21c - Engine RPM in hundreds, also updates once per second 22c - ?
23c - ?
24c - Sun Sensor, Exterior Lights Switch, and 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 (255max.) (=0Xff) 31c - Timing Counter 32c - Displays Test 33c - Software Version (usually 3.4) 34c - ?
35c - Outside Temp from Inlet Sensor (filtered) 36c - Temp