Abstract: A multi-cylinder group engine system operable in at least a first mode and a second mode, where in the first mode a first and second cylinder group combust air and fuel with a lean air-fuel ratio, and where in the second mode at least one of the first and second cylinder groups combusts air and fuel and the other one of the first and second cylinder groups pumps air without injected fuel, the engine system comprising of a fuel injection activity sensor coupled to each cylinder in the first and second cylinder groups; a exhaust gas sensor disposed in an exhaust passage to measure air fuel exhausted from the engine; and a controller configured to transition out of the first mode responsive to detection of exhaust gas sensor degradation and to transition out of the second mode responsive to detection of fuel injection sensor degradation. In one example, the transition out of the second mode may be slower than the transition out of the first mode in response to the respective degradation.