In 2013 the EPA yet again mandated stricter emission controls on diesel engines. The 13-18 6.7 Cummins is equipped with a Exhaust gas recirculation (EGR), Diesel particulate filter (DPF), multiple catalytic converters (CAT) & Selective catalytic reduction (SCR). The 2013+ now requires the use of DEF fluid to aid in reducing emissions. When your truck is relatively new with low mileage these systems function as intended,. Unfortunately with increased mileage fuel economy starts to get worse, DPF regen no longer works like it use to and your faced with poor drivability. As your DPF & CAT get plugged with soot your fuel mileage is also reduced. In addition to poor fuel economy, turbo failure is rather common because of the EGR system. In an attempt to remedy this dealers will recommend cleaning out the DPF, this quite honestly is just a band aid fix. In a few thousand miles you will be right back where you started with poor fuel mileage and a turbocharge waiting to fail. The best thing you can do for your 2013-2018 6.7 Cummins is a complete DPF & EGR delete.