If the exhaled gas concentration levels are constant and within the range for room air with the volunteer attached, check to ensure the ML206 Gas Analyzer's
air pump switch is set to "on", the unit's pump light is lit, and the pump can be heard running. Next, ensure there are no leaks in the gas sampling circuit. Finally, adjust the pump's flow rate, and see if the gas concentration readings respond. If the pump is running and your values are not changing, a mechanical blockage in the gas sampling circuit is likely.
The most common location for a blockage is the outlet port on the back of the gas analyzer; also, the tubing in or out of the desiccant cartridge may have become kinked.