Patent ID: 6931838

Claim:
An air-fuel ratio control method for an internal combustion engine, the engine burning an air-fuel mixture of air and fuel injected from a fuel injector and discharging combustion gas from an exhaust passage through a three-way catalytic converter, the method comprising: detecting an oxygen concentration of the exhaust passage upstream of the three-way catalytic converter; detecting an oxygen concentration of the exhaust passage downstream of the three-way catalytic converter; detecting a running region of the engine; calculating a correction value based on the oxygen concentration of the exhaust passage downstream of the three-way catalytic converter; correcting a preset basic control constant by the correction value as a corrected basic control constant; feedback controlling a fuel injection amount of the fuel injector based on the oxygen concentration of the exhaust passage upstream of the three-way catalytic converter and the corrected basic control constant to cause an air-fuel ratio of the air-fuel mixture to be equal to a stoichiometric air-fuel ratio; and determining that there is an exhaust gas leak in the exhaust passage upstream of the three-way catalytic converter when a difference of the correction values under different running regions exceeds a predetermined value.