High Co2 reading

The Infors minitron is showing a high Co2 reading. The door has been opened and left open and the Co2 gas turned off at the cylinder but still shows on the display that the Co2 is over 10%. As this shows that the amount of Co2 is too much it cannot be used with th egas as it will not allow any gas into it. anyone have any ideas on how to correct this? Thanks

Equipment: Infors HT minitron