and which causes for example the secondary air valve to remain engaged or by retaining the output of the operational amplifier 7 at a fixed potential it causes the mixture controller of the fuel injection system to retain open loop control and to adapt itself to an average fuel mixture in any case the circuitry which follows the operational amplifier 7 recognizes the fixed output potential from the operational amplifier 7 and thus recognizes that the sensor is not yet operational the circuitry then takes the necessary steps to maintain satisfactory and orderly engine operation without having actual information regarding the exhaust gas composition as the engine operates the u03bb sensor is eventually warmed up by the hot exhaust gases and its internal resistance drops thereby also dropping the base voltage at the transistor t2 once the internal resistance 3 has reached a certain value which is determined by the dimensions of the remaining bridge resistors the base voltage of the transistor t2 goes lower than that of the transistor t1 thereby blocking the transistor t2 and hence also the transistor t3 thus at the same time the current flowing through this branch of the bridge is interrupted as a consequence the current i p flowing through the sensor is also interrupted and the entire control system behind the sensor 2 3 is permitted to operate normally and the only current flowing through the sensor will now be the very small input current to the threshold switch equal to the base current of the transistor t4 a control system of this type must be capable of satisfactory operation even when the engine.