Method for diagnosing a failure of the EGR circuit of an engine comprising an EGR exchanger, an EGR valve, a bypass duct of the EGR exchanger, and a so-called bypass flap, arranged upstream of the EGR exchanger and of the bypass duct so as to control the proportion of exhaust gases passing through the latter, wherein the EGR circuit can be activated according to a so-called cooled mode, in which the flap is closed, and a so-called bypass mode, in which the flap is open, characterized in that it comprises the following steps: - estimation of the temperature of the exhaust gases (TsEGR est□byp) at the outlet of the EGR exchanger when the EGR circuit is in bypass mode, - measurement of the temperature of the exhaust gases (TsEGR mes) at the outlet of the EGR exchanger, - calculation of the difference between said estimated temperature (TsEGR est□byp) and said measured temperature (TsEGR mes), - comparison of said difference with a first (S bm) or a second (S cm) predetermined threshold so that: • if the EGR circuit is in bypass mode and the difference is greater than the first threshold (S bm), then a blocking of the flap in cooled mode is detected, • if the EGR circuit is in cooled mode and the difference is less than the second threshold (S cm), then a blocking of the flap in bypass mode is detected, and in that, to estimate the temperature (TsEGR est□byp) of the exhaust gases at the outlet of the exchanger when the EGR circuit is in bypass mode, three efficiencies are calculated, as a function of the flow rate of the exhaust gases (Q EGR), given by the following formulae: • ε 1 Q EGR = T int − Tavt Tco − Tavt • ε 2 Q EGR = TeEGR − Tavt T int − Tavt • ε 3 Q EGR = TeEGR − TsEGR TeEGR − Tco • in which: • Tint is the temperature of the exhaust gases at the inlet of the EGR circuit, • Tco is the temperature of the coolant of the EGR exchanger, • Tavt is the temperature of the exhaust gases upstream of the EGR circuit, • TeEGR is the temperature of the exhaust gases at the inlet of the EGR exchanger. - estimation of the temperature of the exhaust gases (TsEGR est□byp) at the outlet of the EGR exchanger when the EGR circuit is in bypass mode, - measurement of the temperature of the exhaust gases (TsEGR mes) at the outlet of the EGR exchanger, - calculation of the difference between said estimated temperature (TsEGR est□byp) and said measured temperature (TsEGR mes), - comparison of said difference with a first (S bm) or a second (S cm) predetermined threshold so that: • if the EGR circuit is in bypass mode and the difference is greater than the first threshold (S bm), then a blocking of the flap in cooled mode is detected, • if the EGR circuit is in cooled mode and the difference is less than the second threshold (S cm), then a blocking of the flap in bypass mode is detected, • if the EGR circuit is in bypass mode and the difference is greater than the first threshold (S bm), then a blocking of the flap in cooled mode is detected, • if the EGR circuit is in cooled mode and the difference is less than the second threshold (S cm), then a blocking of the flap in bypass mode is detected, • in which: • Tint is the temperature of the exhaust gases at the inlet of the EGR circuit, • Tco is the temperature of the coolant of the EGR exchanger, • Tavt is the temperature of the exhaust gases upstream of the EGR circuit, • TeEGR is the temperature of the exhaust gases at the inlet of the EGR exchanger.