oxidation zone 14 and no 2 generation zone 16 the hc is oxidized before no 2 is generated because the hc oxidizes at a much lower temperature when the hc has been removed from the system the no 2 generation zone 16 can begin to generate no 2 from no in the emissions gases the mixer 28 may be implemented in the exhaust assembly 10 if active urea injection is used to supply nh 3 to the scr system 18 the no x sensor 30 placed before each scr system 18 determines the quantity of no in the emission gas the temperature sensor 22 placed before each scr system 18 determines the temperature of the emission gas entering the scr system 18 referring now to fig2 the control module 36 includes a catalyst control module 38 a diagnostic module 40 and a fuel control module 42 the diagnostic module 40 receives signals from the no x sensors 30 and temperature sensors 22 to determine the level of no and the temperature of the gases within the exhaust assembly 10 the diagnostic module 40 compares the temperature of the coolant with a first predetermined temperature threshold for example only 300 u00b0 c to establish whether catalyst operation is optimal catalyst operation is optimal when the temperature of the coolant is greater than the first predetermined temperature threshold the diagnostic module 40 provides a signal to the catalyst control module.