A system, comprising: an exhaust aftertreatment system configured to treat emissions from a combustion engine, wherein the exhaust aftertreatment system comprises: a three-way catalyst assembly having an inlet; an ammonia slip catalyst assembly configured to receive treated exhaust gases from the TWC assembly and to convert ammonia (NH 3) within the treated exhaust gases into nitrogen (N 2), wherein the ASC assembly has an inlet and an outlet; at least one oxygen (O 2) sensor disposed upstream of the inlet of the TWC assembly and configured to determine a concentration of O 2 in exhaust gases generated by the combustion engine; at least one nitrogen oxides (NO X) sensor disposed downstream of the outlet of the ASC assembly and configured to measure a concentration of NO X in the treated exhaust gases exiting the outlet of the ASC assembly; and a controller configured to receive a first signal from the at least one NO X sensor representative of the concentration of NO X in the fluid exiting the outlet of the ASC assembly and a second signal from the at least one O 2 sensor representative of the concentration of O 2 in the exhaust gases generated by the combustion engine, and to determine whether the catalytic activity has been deactivated in the ASC assembly based at least on the received first and second signals, and characterised in that the controller is configured to determine whether the catalytic activity in the ASC assembly has been deactivated by determining if the concentration of NO X in the treated exhaust gases exiting the outlet of the ASC assembly is greater than a NO X threshold value. an exhaust aftertreatment system configured to treat emissions from a combustion engine, wherein the exhaust aftertreatment system comprises: a three-way catalyst assembly having an inlet; an ammonia slip catalyst assembly configured to receive treated exhaust gases from the TWC assembly and to convert ammonia (NH 3) within the treated exhaust gases into nitrogen (N 2), wherein the ASC assembly has an inlet and an outlet; at least one oxygen (O 2) sensor disposed upstream of the inlet of the TWC assembly and configured to determine a concentration of O 2 in exhaust gases generated by the combustion engine; at least one nitrogen oxides (NO X) sensor disposed downstream of the outlet of the ASC assembly and configured to measure a concentration of NO X in the treated exhaust gases exiting the outlet of the ASC assembly; and a controller configured to receive a first signal from the at least one NO X sensor representative of the concentration of NO X in the fluid exiting the outlet of the ASC assembly and a second signal from the at least one O 2 sensor representative of the concentration of O 2 in the exhaust gases generated by the combustion engine, and to determine whether the catalytic activity has been deactivated in the ASC assembly based at least on the received first and second signals, and characterised in that the controller is configured to determine whether the catalytic activity in the ASC assembly has been deactivated by determining if the concentration of NO X in the treated exhaust gases exiting the outlet of the ASC assembly is greater than a NO X threshold value. a three-way catalyst assembly having an inlet; an ammonia slip catalyst assembly configured to receive treated exhaust gases from the TWC assembly and to convert ammonia (NH 3) within the treated exhaust gases into nitrogen (N 2), wherein the ASC assembly has an inlet and an outlet; at least one oxygen (O 2) sensor disposed upstream of the inlet of the TWC assembly and configured to determine a concentration of O 2 in exhaust gases generated by the combustion engine; at least one nitrogen oxides (NO X) sensor disposed downstream of the outlet of the ASC assembly and configured to measure a concentration of NO X in the treated exhaust gases exiting the outlet of the ASC assembly; and a controller configured to receive a first signal from the at least one NO X sensor representative of the concentration of NO X in the fluid exiting the outlet of the ASC assembly and a second signal from the at least one O 2 sensor representative of the concentration of O 2 in the exhaust gases generated by the combustion engine, and to determine whether the catalytic activity has been deactivated in the ASC assembly based at least on the received first and second signals, and characterised in that the controller is configured to determine whether the catalytic activity in the ASC assembly has been deactivated by determining if the concentration of NO X in the treated exhaust gases exiting the outlet of the ASC assembly is greater than a NO X threshold value.