A system comprising: a catalyst; an emission control module configured to determine an air-fuel ratio and to control at least in air-fuel regulation to operate an engine at the air-fuel ratio; a first oxygen sensor configured to detect oxygen content of gases entering the catalyst and to report the oxygen contents of the gases entering the catalyst to the emissions control module; a second oxygen sensor configured to detect oxygen content of gases exiting the catalyst and to report the oxygen contents of the gases exiting the catalyst to the emissions control module; wherein the emissions control module is configured to determine a first air-fuel ratio based on the contents of the gases entering the catalyst based on the signal from the first oxygen sensor; and to determine a second air-fuel ratio by modifying the first air-fuel ratio based on the contents of the gases exiting the catalyst at least based on the signal from the second oxygen sensor, and to control an air-fuel regulator (241,242) to operate an engine at the air-fuel ratio using the second air-fuel ratio; the system being characterized in that it further comprises: a carbon monoxide sensor configured to detect carbon monoxide content of gases exiting the catalyst and to report the carbon monoxide contents of the gases exiting the catalyst to the emissions control module; and that the determination of the second air-fuel ratio is further modifying the first air-fuel ratio based on the signal from the carbon monoxide sensor. a catalyst; an emission control module configured to determine an air-fuel ratio and to control at least in air-fuel regulation to operate an engine at the air-fuel ratio; a first oxygen sensor configured to detect oxygen content of gases entering the catalyst and to report the oxygen contents of the gases entering the catalyst to the emissions control module; a second oxygen sensor configured to detect oxygen content of gases exiting the catalyst and to report the oxygen contents of the gases exiting the catalyst to the emissions control module; wherein the emissions control module is configured to determine a first air-fuel ratio based on the contents of the gases entering the catalyst based on the signal from the first oxygen sensor; and to determine a second air-fuel ratio by modifying the first air-fuel ratio based on the contents of the gases exiting the catalyst at least based on the signal from the second oxygen sensor, and to control an air-fuel regulator (241,242) to operate an engine at the air-fuel ratio using the second air-fuel ratio; the system being characterized in that it further comprises: a carbon monoxide sensor configured to detect carbon monoxide content of gases exiting the catalyst and to report the carbon monoxide contents of the gases exiting the catalyst to the emissions control module; and that the determination of the second air-fuel ratio is further modifying the first air-fuel ratio based on the signal from the carbon monoxide sensor. a carbon monoxide sensor configured to detect carbon monoxide content of gases exiting the catalyst and to report the carbon monoxide contents of the gases exiting the catalyst to the emissions control module; and that the determination of the second air-fuel ratio is further modifying the first air-fuel ratio based on the signal from the carbon monoxide sensor.