DEF was used to meet the next standard of NOx emissions, which was reduced to between 50% and 10% of the standard met on 1/1/2007, depending on the year of the truck. The trucks running EGR and DPF could not meet the new standard and had to utilize EGR, DEF and SCR to further reduce the NOx emissions. So whats coming out of the tail pipe is cleaner but I've not seen any reduction in how clean the EGR is.
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