I did not know that the oxymeter would to that or not. I am not sure, but when he goes to the doctors they always use one. Maybe there is a different one to buy that does measure co2. I did look up info on the oxymmeter and I guess you can tell by the oxygen level. If it is below 90, the co2 level is getting high. But I guess this did not work for you. You are right, they can be off. Usually I can tell by his behavior. He starts acting goofy and talking about nothing that makes any sense.