A vehicle exhaust system includes a first exhaust after-treatment module that receives engine exhaust gases and a second exhaust after-treatment module that is downstream of the first exhaust after-treatment module. A valve is moveable between an open position that blocks flow to the first exhaust after-treatment module such that all exhaust gas flow bypasses the first exhaust after-treatment module and is directed into the second exhaust after-treatment module, a closed position that directs flow into the first exhaust after-treatment module before the flow enters the second exhaust after-treatment module, and a partially open position where one portion of flow is directed into the first exhaust after-treatment module and a remaining portion of flow is directed into the second exhaust after-treatment module. A controller controls movement of the valve between the open, closed, and partially open positions based on at least one of engine flow rate and NOx output.