Air/fuel ratio control system for internal combustion engines, having engine warming-up detecting means

In an air/fuel ratio control system for use with an internal combustion engine having an exhaust system, including an O.sub.2 sensor provided in the exhaust system of the engine for detecting the concentration of oxygen present in exhaust gases emitted from the engine;

• an air/fuel ratio control valve having a valve body position thereof disposed to determine the air/fuel ratio of an air/fuel mixture being supplied to the engine;
• an actuator arranged to drive the air/fuel ratio control valve in response to an output signal generated by the O.sub.2 sensor;
• and a temperature sensor arranged to detect the temperature of engine coolant;
• the combination comprising:
• a first timer circuit adapted to determine a first predetermined period of time as a function of the temperature of engine coolant available at the start of the engine and start counting the first predetermined period of time thus determined upon the start of the engine;
• a circuit arranged to detect the internal resistance of the O.sub.2 sensor and adapted to generate a signal when the internal resistance of the O.sub.2 sensor lowers below a predetermined value;
• a second timer circuit responsive to the signal generated by the internal resistance detecting circuit to start counting a second predetermined period of time;
• and means for causing initiation of air/fuel ratio control operation based upon the output signal of the O.sub.2 sensor, after the first and second timer circuits both have finished counting the first and second predetermined periods of time, respectively.