A diagnostics method for a supercharged internal combustion engine including a fixed-geometry turbocharger, including a compressor through which air admitted into an intake circuit passes, and a turbine rotationally connected to the compressor and through which exhaust gases pass, the method including: calculating a first integral with respect to time of a measurement of atmospheric pressure; calculating an integral with respect to time of a measurement of boost pressure; calculating a second integral with respect to time of a measurement of atmospheric pressure; calculating two diagnostics criteria dependent on the first integral of atmospheric pressure, on the integral of the boost pressure and on the second integral of the atmospheric pressure; comparing the first diagnostics criterion with a first diagnostics threshold and comparing the second diagnostics criterion with a second diagnostics threshold; and diagnosing a fault when at least one of the diagnostics criteria is below its associated diagnostics threshold.