a failure detection system for an air - to - fuel ratio control system includes a exhaust gas monitor positioned upstream of a catalytic converter for monitoring an emission content of exhaust gas based on which an air - to - fuel ratio is feedback controlled so as to be maintained at a desired ratio . the system has another exhaust gas monitor positioned downstream of the catalytic converter for monitoring an emission content of exhaust gas passed through the catalytic converter . functional failures of the exhaust gas monitor of the air - to - fuel ratio control system is judged to be present on the basis of detected emission contents only when a predetermined threshold amount of intake air is detected .