There is disclosed a method for diagnosing a fault condition in an electric machine 4, the electric machine 4 comprising a rotor 10 having an axis of rotation 14 and comprising a plurality of permanent magnets M1-M8. The method comprises: rotating the rotor 10 so that each of the plurality of permanent magnets M1-M8 passes a magnetic flux density sensor 6; measuring values of magnetic flux density using the magnetic flux density sensor 6 at a plurality of positions of the rotor 10; analysing the measured values of magnetic flux density and comparing a magnitude of a peak amplitude in a time or frequency domain with a reference value; and if the peak amplitude is below the reference value, determining that the electric machine 4 has a fault condition.