A gas sensor system includes an ammonia-sensing cell for generating a signal upon exposure to an unknown gas comprising ammonia, an A/F cell for generating a signal upon exposure to hydrocarbons in the gas, a heater in thermal communication with the cells and a housing in which the cells and the heater are mounted. The housing permits an unknown gas to flow therethrough for contact with the cells, and there is a sensor control circuit in communication with the cells. The sensor control circuit is configured to utilize the signals from the cells to generate an ammonia concentration signal indicating the concentration of ammonia in the unknown gas. Ammonia may be sensed in an unknown gas by heating such cells to selected working temperatures, exposing them to an unknown gas, obtaining signals from the cells, and using the cell signals to determine the ammonia content.