A catalytic converter degradation determining system is configured to accurately determine if a NOx trapping catalytic converter is degraded. The system is configured to detect or estimate the temperature of a NOx trapping catalytic converter and sets a pre-degradation adsorption efficiency and a post-degradation adsorption efficiency based on the catalytic converter temperature. The system then multiplies the adsorption efficiencies by an exhaust gas NOx quantity to calculate trapped NOx rates. The individual trapping NOx rates are summed over a prescribed amount of time to estimate the pre-degradation and post-degradation NOx trapping amounts. After the prescribed amount of time elapses, the system compares the pre-degradation NOx trapping amount to the post-degradation NOx trapping amount and determines if the difference between the two is smaller than a prescribed value. If the difference is smaller than the prescribed value, then the system determines that the catalytic converter is degraded.