The only thing DEF does is reduce the NOx in the exhaust. It is sprayed into the exhaust and works with the CAT to clean the exhaust. The other component in the exhaust is the DPF, which traps diesel particulates, and is burned out occasionally by the injection of diesel into the exhaust stream. That is the cycle that produces the bad smell when the system goes into a regen cycle. If the system doesn't do the regen regularly, the DPF gets clogged and the result is a restricted exhaust. The onboard computer will force a regen if the truck isn't run at highway speeds when a regen is called for, and if it isn't performed, the truck will have to go to dealer to be cleaned out or the DPF will have to be replaced. A good site that has a FAQ on DEF is
http://www.discoverdef.com/def-overview/faq/ A Wikipedia on DPF is
http://en.wikipedia.org/wiki/Diesel_particulate_filter