The only way I can think to test the sensor, per se, would be to load an test program onto the Egg using Arduino, and I don't have one handy to point you to at the moment unfortunately.
I would advise that for repairs you should probably send an Egg back to us rather than trying to do an in-place repair / replace operation yourself. For the CO2 I Egg, I believe there are four wires connecting the sensor board to the processing board (rx, tx, gnd, and 5V), and they are soldered to the respective boards on both ends if I recall correctly.