Device for controlling the operating status of a member (15) that treats gaseous effluent from an exhaust line (13) of an internal combustion engine (1) comprising a control unit (30) including a module (31) that verifies the operating conditions of the engine (1), a fuel injection module (32) that injects fuel upstream of the treatment member (15) and a diagnostics module (33) performing diagnostics on the treatment member (15), said diagnostics module (33) comprising a model of the treatment member without catalytic activity and a calculation module (35) calculating a difference between the outlet temperature of said model and a measured value of the outlet temperature of the treatment member (15). The diagnostics module (33) comprises a module that devises a diagnostics criterion dependent on the temperature difference and on an air/fuel mixture richness value measured by an oxygen probe (21) situated upstream of the treatment member (15).