A method for diagnosing a failure of an EGR circuit of an engine including an EGR exchanger, an EGR valve, a bypass duct of the EGR exchanger, and a bypass flap arranged upstream from the EGR exchanger and the bypass duct to adjust a proportion of exhaust gases flowing therethrough. The EGR circuit can be activated according to a cooled mode, in which the flap is shut, or according to a bypass mode, in which the flat is open. The method carries out, during a diagnosis phase, two activations of the flap, and measures an average of variations of the EGR gas temperature at an outlet of the EGR exchanger during the diagnosis phase.