A method for monitoring and diagnosing an air charger system for use in an engine assembly having pre-existing sensors, having the steps of providing an air charger having a turbine and a compressor operatively coupled to one another. Providing a control unit for monitoring a plurality of operating conditions of the engine assembly. The control unit calculates an expected value of a selected operating condition based upon at least another of the plurality of operating conditions. Comparing the expected value and at least another of the plurality of operating conditions. Diagnosing a fault condition, if present, based upon the expected value and said at least another of said plurality of operating conditions being outside a predetermined tolerance.