Distinction between the EGR valve and the oxygen sensor function reduction

The invention relates to a diagnostic method for an engine with a valve for controlling the dilution of intake air. The method comprises, while the engine fuel is supplied, displaying a loss of function of the valve when the valve is closed, the sensor output but an oxygen content equivalent to less than that of air. The method further includes, during the engine, no fuel is supplied, displaying a loss of function of the sensor when the valve is closed, but the sensor output corresponding to an oxygen content below that of air.