Abstract: An internal combustion engine fuel ignition system has feedback from an exhaust gas composition transducer which operates effectively only at normal exhaust temperatures, the transducer having a very high impedance at low temperatures. The feedback circuit includes a comparator stage and an integrator stage integrating the output of the comparator stage which compares the transducer output with a reference voltage. A feedback resistor is connected from the integrator stage output to the comparator stage input, so that, at low temperature the integrator stage output stabilises at a predetermined level. Only when the transducer warms up so that its impedance becomes significantly less than that of the feedback resistor does the exhaust feedback control loop become effective.