Case 621G - Page 236

7-MAINTENANCE
RemovalofNO
x
requiresachemicalreaction.Thischem-
icalreactiontakesplaceintheSCR(13).DEF/ADBLUE®
reactswiththeSCRcatalysttoconvertNO
x
toharmless
nitrogenandwatervapor.
Toassurethattheemissionsystemisfunctioningcor-
rectly,adownstreamNO
x
sensor(16)islocatedinthe
exhauststreamattheoutletoftheSCR.
TheNH
3
sensor(15),locatedattheoutletoftheSCRcat-
alystmonitorsammonia(NH
3
)levelstoensuretheproper
amountofDEF/ADBLUE®isbeingused.
Temperatureisveryimportanttothechemicalreactions
thattakeplaceintheemissioncontrolsystem.AnSCR
upstreamtemperaturesensor(12)islocatedintheex-
hauststreamattheinletoftheSCRandcomparesthe
readingoftheSCRdownstreamtemperaturesensor(14)
locatedintheexhauststreamattheoutletoftheSCR.The
catalystrequiresabout230°C(446°F)forthechemical
reactiontotakeplace.
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