Air/fuel ratio feedback control system for internal combustion engines, having atmospheric pressure-dependent fail safe function for O2 sensor

Claim 1

An air/fuel ratio feedback control system for combination with an internal combustion engine, comprising: first sensor means for detecting the concentration of oxygen present in exhaust gases emitted from said engine; valve means having a valve body disposed to determine the air/fuel ratio of an air/fuel mixture being supplied to said engine; electronic control means operable in response to an output signal of said first sensor means to drive said valve means, whereby the air/fuel ratio of said mixture is controlled to a predetermined value in a feedback manner responsive to changes in the concentration of oxygen present in exhaust gases emitted from said engine; means adapted to generate a first signal as long as a predetermined condition for effecting said feedback control of the air/fuel ratio of said mixture is fulfilled; means adapted to generate a second signal as long as said first sensor means is activated; means adapted to determine an actual air/fuel ratio of said mixture from the value of said output signal of said first sensor means and to generate a third signal having a binary value invertible depending upon whether the air/fuel ratio thus determined is larger or smaller than said predetermined value; safety means arranged to be supplied with said first, second and third signals for performing a predetermined safety action when no inversion ocurs in said third signal inputted thereto for a predetermined period of time while simultaneously said first and second signals are both inputted thereto; second sensor means for detecting ambient atmospheric pressure ; and means adapted to render said safety means inoperative when a value of ambient atmospheric pressure detected by said second sensor means is lower than a predetermined value.