Vehicle bypass catalyst diagnostic system

A diagnostic method for a bypass catalytic converter of 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 the 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 an oxygen sensor disposed downstream of the bypass catalytic converter, the diagnostic method comprising:
• flowing exhaust gas through the light-off catalyst bypass system;
• monitoring, by the controller, signals from the downstream oxygen sensor for a predetermined time period during an engine cold start condition;
• determining, by the controller, a curve plotting oxygen content at the downstream oxygen sensor over the predetermined time period, based on the signals from the downstream oxygen sensor;
• calculating, by the controller, an accumulated area under the curve over the predetermined time period;
• and
• comparing, by the controller, the calculated accumulated area to a predetermined threshold to determine if the bypass catalytic converter has failed.