Do the following steps to retrieve all the codes stored in your C5:
1) Turn your ignition on. The engine doesn't have to be running, but it's ok if it is...
2) Press reset to clear any warning messages
3) Press and hold the "options" button
4) Press the "fuel" button 4 times while holding the options button down. The car will enter manual diagnostics and start displaying all the codes that are set for every control module in the car.
The diagnostic codes will flash across the screen in the following categories:
PCM - Poweretrain
TCS- Traction control and ABS
RTD- Real Time dampening
BCM- Body Control Module
IPC- Instrument Panel Cluster
HVAC- Heating, ventilation and A/C
LDCM- Left Door Control module
RDCM- Right Dorr control module
SCM- Seat control module
RFA- Remote function actuation
"C" after the code denotes that it is within the current ignition cycle.
"H" means it has occured in previous ignition cycles, but not the current one.
When you get the codes you can post them up and we'll see if we can help