Diagnostics method for diagnosing a nitrogen oxides trap housed in a treatment device that treats the pollutant emissions of an engine, downstream of the trap, said trap being said device housing an additional anti-pollution system downstream of the trap, said trap being able, during a regeneration phase, to break down the nitrogen oxides stored in the trap by reaction with reducing agents, said method comprising: - a step of determining a diagnostics criterion dependent on the mass of reducing agents (M red) consumed during the regeneration phase; - a step of comparing the diagnostics criterion with a defectiveness threshold below which it is determined that the trap is defective; and - a step of emitting a signal indicating the state of defectiveness of the trap, CHARACTERIZED IN THAT the diagnostics criterion is determined as being equal to the product: - of the mass of reducing agents (M red) consumed during the regeneration phase, calculated as the integral, with respect to time, for the duration of the regeneration phase (Δt), of the difference between a mass flow rate (Q us) of reducing agents upstream of the device and a mass flow rate (Q ds) of reducing agents downstream of the device; - of the reduction efficiency (ε red) of the trap during the regeneration phase, dependent on the temperature (θ) of the trap during the regeneration phase; and - of a function (f) of the duration (Δt) of the regeneration phase, according to the equation: f Δt = min T min, Δt / max T max, Δt in which equation - min(T min,Δt) denotes the smallest value out of a predetermined minimum duration (T min), equal to the time taken for the exhaust gases from the engine to transfer through the additional anti-pollution system, and the regeneration duration (Δt); - max(T max,Δt) denotes the largest value out of a predetermined maximum duration (T max) and the regeneration duration (Δt). - a step of determining a diagnostics criterion dependent on the mass of reducing agents (M red) consumed during the regeneration phase; - a step of comparing the diagnostics criterion with a defectiveness threshold below which it is determined that the trap is defective; and - a step of emitting a signal indicating the state of defectiveness of the trap, - of the mass of reducing agents (M red) consumed during the regeneration phase, calculated as the integral, with respect to time, for the duration of the regeneration phase (Δt), of the difference between a mass flow rate (Q us) of reducing agents upstream of the device and a mass flow rate (Q ds) of reducing agents downstream of the device; - of the reduction efficiency (ε red) of the trap during the regeneration phase, dependent on the temperature (θ) of the trap during the regeneration phase; and - of a function (f) of the duration (Δt) of the regeneration phase, according to the equation: f Δt = min T min, Δt / max T max, Δt in which equation - min(T min,Δt) denotes the smallest value out of a predetermined minimum duration (T min), equal to the time taken for the exhaust gases from the engine to transfer through the additional anti-pollution system, and the regeneration duration (Δt); - max(T max,Δt) denotes the largest value out of a predetermined maximum duration (T max) and the regeneration duration (Δt).