A system or method for controlling a multiple cylinder internal combustion engine operable in a reduced displacement mode with at least one valve or cylinder selectively deactivated include controlling the engine in response to detecting opening of an exhaust valve after deactivation based measured or inferred exhaust pressure changes after deactivation. In one embodiment, controlling the engine includes reactivating one or more deactivated cylinders/valves, inhibiting subsequent deactivation, and/or storing a diagnostic code in a computer readable storage medium in response to detecting valve opening after deactivation.