C1 is short for Confidence level 1, meaning that the primary oxygen sensor produces a response that is outside the allowed parameters, at a sensor validation.
As the sensor gives a response not regarded as correct, the unit chooses not to trust the sensor, thus the alarm.
This also includes the case if a sensor gives the exact same PO2 value at two consecutive validations. Then the unit thinks the primary sensor is frozen. If the PO2 value is changed at the next validation (aprox. one and a half minutes later) the alarm will stop.
All C1 alerts should be handled like a critical alarm and all instructions on the display should be followed.