Sensing particular gases in a mixture uses precise modulated heating. Sensor relative responses are compared at different temperatures and compared with known relative responses to identify gases and concentrations. Heater current sensors provide feedback control and microprocessor inputs. A processor controls complex impedances and varied frequencies in the sensors. Sensor responses at varied complex impedances and at varied frequencies are compared with known responses at those impedances and frequencies to determine existence and concentration of particular gases. Heater and sensor buses are separate or combined.