A method for monitoring the concentration of a gas in an enclosed space is provided. A gas is injected into the space, by a gas injection system. The concentration of the gas is measured using a gas sensor. The concentration is compared with predetermined limits, and the gas injection system is shut off if the concentration is above one of the predetermined limits. In addition, hardware, software, and signal control elements which perform the monitoring, are continuously monitored for faults, and if a fault is detected, the gas injection system is shut off.
