is determined in step 208 whether npdd 56 power is zero if so control passes to step 210 in which it is registered that npdd 56 is not working if a negative result from step 208 control passes to step 202 to further reduce npdd 56 power the power is progressively reduced until npdd 56 power is turned off the purpose of progressively lowering power to npdd 56 as shown in the loop of steps 202 204 and 208 is to ensure that npdd 210 is truly not working in one scenario if the power to npdd 56 is higher than need be then dropping the power does not result in a measurable difference in nox concentration at the exit of lnt 58 thus to obtain an accurate determination of nox concentration this possible scenario is ruled out by steps 202 204 and 208 an alternative to the diagnostic method of fig4 is to instead turn off power to npdd 56 in step 202 i e turn it off completely rather than progressively reduce npdd 56 power steps 208 and 214 are unnecessary in the alternative if a negative result is returned from step 204 control passes directly to step 210 the difference between fig4 and the alternative to fig4 is that by turning off npdd 56 completely the no to no2 conversion is completely turned off during even a short interval in which no is not converted to no2 no breaks through exhaust aftertreatment system.