Abstract: A system for monitoring suction leakage of external air into the exhaust gas path of an internal combustion engine during intermittent reduced pressure phases of the engine, during operation, includes an oxygen concentration sensing lambda probe arranged in the path. The probe issues an output signal having a reduced pressure phase pulsation frequency component of reduced pressure phase amplitude indicative of such oxygen concentration during the reduced pressure phases. The output signal is fed to a control device which generates an error signal when that amplitude exceeds a predetermined threshold level indicative of suction entry of external air into the path during a reduced pressure phase.

Abstract: Apparatus for monitoring suction leakage of external air into the exhaust gas path of an internal combustion engine during intermittent reduced pressure phases of engine running, includes two oxygen concentration sensing lambda probes arranged at a distance from one another along the path. The probes issue signals indicative of such oxygen concentration which are fed to a control device, such that an error signal is generated on deviation of the two probe signals from one another as caused by suction entry of external air into the path solely during a reduced pressure phase.