Method for detecting a fault in a system comprising an internal combustion engine and a turbocompressor

The method involves determining power of a compressor (11) from parameters of air flowing through an intake pipe (3) of an internal combustion engine (2) e.g. diesel or petrol engine, and power of a turbine (10) based on parameters of combustion gas flowing in an exhaust pipe (4). Speed of a turbocharger (9) is determined by interpolation of compressor field of the turbocharger. Another speed of the turbocharger is determined from power balance of a shaft (12) of the turbocharger by comparison of the powers. A defect in a system is detected from comparison of two values of the speeds.