A method for introducing a reagent medium into an exhaust gas conduit of an internal combustion engine (10), and an apparatus for carrying out the method, are proposed. Provision is made for a diagnosis of the pressure sensor (20, 22). In at least one first time interval (t8, t10) in which a steady pressure state is expected, a check is made as to whether the pressure signal (21, 23) at least approximately corresponds to the steady-state pressure (100, 107). In at least one second time interval (t9, t11) in which a pressure change occurs, a check is made as to whether the pressure signal (21, 23) at least approximately corresponds to a predetermined test pressure (104, 108) and/or at least approximately exhibits a predetermined change with respect to the steady-state pressure (100, 107). A fault signal (72) is supplied if at least one of the conditions is not met.