Abstract: A CO based, Diesel discriminating, fire detection system and method is disclosed which significantly reduces the threshold of CO needed to detect a fire in a mine that also contains CO from Diesel equipment without a corresponding increase in the number of false alarms. The system measures both the CO and NO levels at many points throughout the mine and calculates a CO/NO ratio for each of these locations. A long term average concentration (4 hours) and a short term average concentration (1 hour) for both CO and NO are continually updated to reflect the changing dynamics of the mine environment. Using the CO/NO ratio, the detection system can effectively differentiate between CO produced by a Diesel and CO produced from a fire and can activate an alarm when the CO level resulting from a fire exceeds a predetermined threshold level.