DEF ( Diesel Exhaust Fluid ) fluid as used in newer truck tractors & straight trucks ( anything over 8000 LBS GVW I recall ) is sprayed onto the SCF ( Selective Catalytic Filter ), which burns off much of the remaining bad exhaust gasses. HIGH HEAT is the operative word here; next time you drive alongside a big truck, look for the HOT label stamped into the tin box under the passenger door ( on most trucks ). Two big issues :
1) You canNOT let the truck run out of DEF; it will do MAJOR damage to the emissions system. Most engines will shut down before DEF runs out anyways.
2) The SCF filters have only half the life span ( appx. 300,000 miles ) that truck builders predicted. Excessive idling and part - throttle operation ( i.e. - in traffic ) is harder on the system than sustained full - throttle ( open road ) operation. The SCF filter is a HIGH - dollar item to replace; even cleaning them is costing more than many trucking companies want to pay. Many companies are turning over their fleets just before the SCF filters need this expensive service. ( Buyer Beware ) DEF quality is also an issue; there is a difference the system notices.
My daily work truck requires a Parked Re - Gen from time to time as indicated on the instrument panel. I have to allow 45 - 50 minutes for this; the system runs the engine at FULL THROTTLE during Re - Gen. Kind of a counter - productive way to reduce emissions in my opinion.