Vehicle bypass catalyst diagnostic system

An internal combustion engine system comprising:

• an internal combustion engine;
• a main exhaust aftertreatment system with a main catalytic converter configured to receive exhaust gas from the internal combustion engine;
• 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;
• and
• an emissions control system including a controller in signal communication with a first temperature sensor disposed upstream of the bypass catalytic converter, and a second temperature sensor disposed downstream of the bypass catalytic converter, wherein the emissions control system is configured to perform a diagnostic of the bypass catalytic converter to determine if the bypass catalytic converter meets a predetermined conversion efficiency, comprising:
• monitoring, by the controller, signals from the first and second temperature sensors for a predetermined time period during an engine cold start condition;
• determining, by the controller, a temperature of an inlet of the bypass catalytic converter, based on the signals from the first temperature sensor;
• determining, by the controller, a temperature of an outlet of the bypass catalytic converter, based on the signals from the second temperature sensor;
• and
• comparing, by the controller, the bypass catalytic converter outlet temperature to the bypass catalytic converter inlet temperature over the predetermined time period to determine if the bypass catalytic converter has failed,
• wherein if the bypass catalytic converter outlet temperature exceeds the inlet temperature during or at the end of the predetermined time period, the controller sets the bypass catalytic converter as passing, thereby indicating a healthy bypass catalyst conversion efficiency;
• and
• wherein if the bypass catalytic converter outlet temperature is less than or equal to the inlet temperature during or at the end of the predetermined time period, the controller sets the bypass catalytic converter as failing, thereby indicating an inefficient catalyst conversion efficiency.