CareFusion MicroCO Operating Manual page 20

The unit must not be used, however, for one second after the unit is
first switched on i.e. before the countdown commences. Expired
alveolar gas is then entrapped between sensor and mouthpiece valve.
The display reading will rise to a plateau over the course of a several
seconds. The final value will be held until the unit is turned off and
represents parts per million CO or %COHb depending on the slide
switch position.
The lights will come on according to the table on page 12 and the red
light will flash and an alarm will sound if the measurement rises above
72ppm (12%COHb).
If this occurs then the possibility of CO poisoning should be
investigated as this level of CO is unlikely to be produced by cigarette
smoking.
If the unit is switched on again too quickly after use there may be a
response to residual expired carbon monoxide from the previous test.
In this case the display will show:
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