I am no expert, but as nobody has chimned in yet, I give it a try.
You have to give more info on the car (year) for the experts, as the injection system might differ.
In general, when the car is warmed-up, the CO2 sensor is measuring how much air is left unburnt in the exhaust. The system tries to match fuel and air as best as possible in order to avoid "dirty" exhaust gases. It does that by fluctuating around too rich and too lean.
The fact that you have it fluctuating tells me that the system is working.
I would suspect that spark plugs or wires or distributor rotor are more likely to be responsible for a rough idle. How old are they?
There are also many previous threads about reading duty cycle, so a search might get you faster results.
Hope this helps.