Abstract: An internal combustion engine system includes an internal combustion engine with a plurality of combustion chambers each having a main exhaust valve and a cold start exhaust valve, a main exhaust aftertreatment system with a main catalytic converter configured to receive exhaust gas from the internal combustion engine via the main exhaust valves and an exhaust manifold, and a light-off catalyst bypass system with a bypass passage and a bypass catalytic converter configured to selectively receive exhaust gas from the internal combustion engine via the cold start exhaust valves. A controller is programmed to determine a cold start, long idle, and/or low main catalytic converter temperature condition, deactivate the main exhaust valves to facilitate preventing exhaust gas flow through the exhaust manifold, and activate the cold start exhaust valves to enable exhaust gas flow through the bypass passage and the bypass catalytic converter.