Air/fuel ratio control method having fail-safe function for abnormalities in oxygen concentration detecting means for internal combustion engines

A method for controlling the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine having a means for detecting oxygen concentration in exhaust gases emitted from the engine, to desired values in such a manner as to correct a basic value of fuel supply quantity for the engine by the use of a first coefficient which has a value thereof variable with a change in the output of said oxygen concentration detecting means, while the engine is operating in a feedback mode control region, and by the use of a second coefficient which is a mean value of values of said first coefficient which have been used during operation of the engine in said feedback mode control region, while the engine is operating in each of a plurality of predetermined particular operating regions other than said feedback mode control region, the method comprising the steps of:

• (1) comparing the value of said second coefficient with a predetermined upper limit and with a predetermined lower limit, said predetermined upper limit and said predetermined lower limit being, respectively, an upper limit and a lower limit of values of said first coefficient that can be assumed while said oxygen concentration detecting means is functioning normally when said engine is operating in a normal operating condition, (2) setting the value of said second coefficient to a predetermined value intermediate between said predetermined upper limit and said predetermined lower limit and controlling the air/fuel ratio of the air/fuel mixture being supplied to the engine by the use of said predetermined value as said second coefficient, when it is determined in the step (1) that the value of said second coefficient is either larger than said predetermined upper limit or smaller than said predetermined lower limit, while said engine is operating in said each of said predetermined particular operating regions, thereby enabling positive continuation of the operation of the engine without causing stoppage of the engine.