Oxygen sensor monitor apparatus

For use in association with a fuel mixture preparation system of an internal combustion engine which uses closed-loop control based on exhaust chemistry as determined by an oxygen sensor, there is described a circuit which monitors the operational readiness of the oxygen sensor on the basis of its internal resistance. The measurement takes place in a bridge circuit which is supplied with current whenever the sensor is non-operative but which becomes inactive when the sensor is at normal temperatures. The bridge voltages are fed to a comparator which generates an average output value when the oxygen sensor is not yet ready for operation and this average signal is processed in an integrator for final control of the fuel-air mixture.