Question please . What is a urea tank or def ??
Per eHow.com:
Urea Tanks for Diesel Engines
"Urea tanks will be mandatory on all diesel engines made in the United States starting in 2010.
Urea binds with the nitrogen oxide emitted by diesel engines, thus reducing diesel emissions responsible for smog and respiratory illnesses such as asthma.
According to a spokesperson from the Selective Catalytic Reduction Stakeholders Group, urea tanks reduce the nitrogen oxide produced by diesel engines by as much as 90 percent.
Use of a diesel particulate filter can further reduce emissions to almost 100 percent."
Per Fleetguard.com:
DEF (Diesel Exhaust Fluid)
"DEF is a urea-based chemical reactant designed specifically for use in SCR systems to reduce NOx emissions."
So DEF goes into the Urea Tank and makes the world a better place.
I hope that answers your question!