Method for diagnosing a nitrogen oxide trap of an internal combustion engine, the said trap being able, during a purge step, to break down the nitrogen oxides stored in the trap by reaction with fuel from the engine, comprising: - a step of determining the richness at the inlet (Ram) and at the outlet (Rav) of the trap for the duration (Δt) of the purge; - a step of calculating at least one diagnostic criterion from a set of parameters including at least the richness at the inlet (Ram) and/or the richness at the outlet (Rav); and - a step (900, 1000) of judging whether the trap status is good or defective on the basis of the comparison of the said diagnostic criterion with a threshold, characterized in that it further comprises: - a step of determining a filtered value for richness at the inlet (Ram,f) and at the outlet (Rav,f) of the trap for the duration of the purge; - a step of calculating: i. a first diagnostic criterion (Cam) equal to the integral with respect to time, over the duration (Δt) of the purge, of the absolute value of the difference between the richness at the inlet (Ram) of the trap and the filtered richness (Ram,f) at the inlet of the trap, and ii. a second diagnostic criterion (Cav), equal to the integral with respect to time, over the duration (At) of the purge, of the absolute value of the difference between the richness at the outlet (Rav) of the trap and the filtered richness (Rav,f) at the outlet of the trap; - a step of comparing the first diagnostic criterion (Cam) with a first threshold (Sam); - a step of comparing the second diagnostic criterion (Cav) with a second threshold (Sav); and - a step of judging that the trap status is defective when the first criterion (Cam) is above the first threshold (Sam) and the second criterion (Cav) is above the second threshold (Sav). - a step of determining the richness at the inlet (Ram) and at the outlet (Rav) of the trap for the duration (Δt) of the purge; - a step of calculating at least one diagnostic criterion from a set of parameters including at least the richness at the inlet (Ram) and/or the richness at the outlet (Rav); and - a step (900, 1000) of judging whether the trap status is good or defective on the basis of the comparison of the said diagnostic criterion with a threshold, - a step of determining a filtered value for richness at the inlet (Ram,f) and at the outlet (Rav,f) of the trap for the duration of the purge; - a step of calculating: i. a first diagnostic criterion (Cam) equal to the integral with respect to time, over the duration (Δt) of the purge, of the absolute value of the difference between the richness at the inlet (Ram) of the trap and the filtered richness (Ram,f) at the inlet of the trap, and ii. a second diagnostic criterion (Cav), equal to the integral with respect to time, over the duration (At) of the purge, of the absolute value of the difference between the richness at the outlet (Rav) of the trap and the filtered richness (Rav,f) at the outlet of the trap; i. a first diagnostic criterion (Cam) equal to the integral with respect to time, over the duration (Δt) of the purge, of the absolute value of the difference between the richness at the inlet (Ram) of the trap and the filtered richness (Ram,f) at the inlet of the trap, and ii. a second diagnostic criterion (Cav), equal to the integral with respect to time, over the duration (At) of the purge, of the absolute value of the difference between the richness at the outlet (Rav) of the trap and the filtered richness (Rav,f) at the outlet of the trap; - a step of comparing the first diagnostic criterion (Cam) with a first threshold (Sam); - a step of comparing the second diagnostic criterion (Cav) with a second threshold (Sav); and - a step of judging that the trap status is defective when the first criterion (Cam) is above the first threshold (Sam) and the second criterion (Cav) is above the second threshold (Sav).