Control systems in which transition between oxidizing and reducing state of the exhaust gases is sensed, and the input to the engine modified in accordance with sensed exhaust gas composition utilize a sensor which has an output signal having a transition jump, the extent of the jump being highly temperature dependent; in accordance with the invention, a signal is derived representative of change in level of the transition jump, as a function of temperature, and the control circuit responsive to the transition jump, and controlling input to the engine is modified in accordance with the signal repesentative of level change of the transition jump, to compensate for temperature-dependent changes.