A furnace system responsive to a thermostat includes a pollutant sensor for sensing a pollutant concentration in the furnace system. The pollutant sensor is configured to open when the pollutant concentration reaches a pollutant threshold and close when the pollutant concentration falls below the pollutant threshold. When the thermostat is calling for heat, a furnace controller monitors the pollutant sensor and disables the furnace system for a lockout period if a lockout criterion related to the pollutant sensor is met.