An exhaust emission control system for an internal combustion engine is disclosed. The exhaust emission control system has a first passageway and a second passageway disposed to receive exhaust in parallel. The exhaust emission control system also has a valve arrangement configured to regulate the flow of exhaust to the first and second passageways, and at least one sensor configured to sense a parameter indicative of a constituent of the exhaust. The at least one sensor generates a signal corresponding to the sensed parameter. The exhaust emission control system also has a controller in communication with the valve and the at least one sensor. The controller is configured to affect operation of the valve in response to the signal.