The present invention is a new sensor system with catalyst for control and one with a dummy catalyst. An apparatus and method for rapidly determining the concentration of CO, water vapor, and temperature. One of the major reasons to rapidly determine a small but sudden rise CO is for fire detection. The combination of the CO, followed by rapid relative humidity rise is a clear indication of possible fire or an appliance malfunction, blocked flu or other combustion process gone awry. If the incident is accompanied by ions or particles that would be smoke from a fire. The signal from the CO determined by numerically compensating for humidity changes. Any one or two parameters could cause an increase the sensitivity of the other in a fire detector such as combination CO, relative humidity, temperature and smoke detector. Another application for rapid CO detection is in a vehicle to protect the occupants from a catastrophic CO event. The preferred embodiment by means of optically monitoring the control and CO responding sensors that respond to the CO by means monitoring the optical change to the humidity in one and humidity and CO in the other and provide a means to remove the humidity.