Air-fuel ratio control system for internal combustion engines

In an air-fuel ratio control system for an internal combustion engine having an exhaust passage, and a catalytic converter arranged in said exhaust passage for purifying noxious components contained in exhaust gases from said engine, said air-fuel ratio control system including an exhaust gas component concentration sensor arranged in said exhaust passage at a location upstream of said catalytic converter for delivering an output based on a pumping current flowing therein and being substantially proportional to concentration of a specific component in said exhaust gases,

• the improvement comprising:
• sensor activation-determining means for detecting an activated state of said exhaust gas component concentration sensor;
• pumping current absence-detecting means for detecting a predetermined pumping current state of said exhaust gas component concentration sensor in which no pumping current flows therein;
• central value-calculating means for calculating a central value of said output from said exhaust gas component concentration sensor to obtain a calculated central value, based on said output from said exhaust gas concentration sensor, when said activated state of said first exhaust gas component concentration sensor is detected and at the same time said predetermined pumping current state of said exhaust gas component concentration sensor is detected, said central value serving as a reference value based on which said exhaust gas component concentration sensor delivers said output;
• output value-correcting means for correcting a value of said output from said exhaust gas component concentration sensor based on said calculated central value to obtain a corrected output value;
• and
• actual air-fuel ratio-calculating means for calculating an actual value of an air-fuel ratio of a mixture supplied to said engine, based on said corrected output value from said exhaust gas component concentration sensor.