If the answer to step 518 is yes, the routine continues to {HEAD ~ step} 520 where {TAIL ~ the routine} sets the desired exhaust and catalyst temperature based on the de SOx conditions 
If the answer to step 518 is yes , the routine continues to { HEAD ~ step } 520 where { TAIL ~ the routine } sets the desired exhaust and catalyst temperature based on the de SOx conditions
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH