Hi AKJ,
I have only used the NDIR type of CO2 sensor because it has fast response, high sensitivity and range, and minimal 'warm up' time. I work in an office where the air typically is 600-800 ppm of CO2. This is a very low level, less than 0.1 percent. If your number of "40" is from digitalRead()-ing an analog pin, then it represents a small number, which is consistent with the amount of CO2 in air. Do an experiment: place the sensor in an enclosure and breath the limited air inside, in order to elevate the CO2 level. Does the go way up?