purging operating condition at step 722 by resetting the purge flag prg_flg to logical zero the controller 14 further initializes a post purge event excess fuel determination by setting a suitable flag xs_fuel_ 2 _calc to logical one returning to steps 710 and 724 of fig7 when the controller 14 determines that the purge flag prg_flg is not equal to logical one and further that the post purge event excess fuel determination flag xs_fuel_ 2 _calc is set to logical one the controller 14 begins to determine the amount of additional excess fuel already delivered to and still remaining in the exhaust system 32 upstream of the trap 36 as of the time that the purge event is discontinued specifically at steps 726 and 728 the controller 14 starts determining a second excess fuel measure xs_fuel_ 2 by summing the product of the difference xs_fuel_rate_stoich by which the first output signal signal 1 is rich of stoichiometry and summing the product of the difference xs_fuel_rate_stoich and the mass air flow rate am the controller 14 continues to sum the difference xs_fuel_rate_stoich until the first output signal signal 1 from the nox sensor 40 indicates a stoichiometric value at step 730 of fig7 at which point the controller 14 resets the post purge event excess fuel determination flag xs_fuel_ 2 _calc at step 732 to logical zero the resulting second excess fuel measure value xs_fuel_ 2 representing the amount of excess fuel exiting the tailpipe 42 after the purge event is discontinued is graphically illustrated as the cross hatched area.