Problem: The truck had a diagnostic trouble code, P20EE (NOx catalyst efficiency below threshold), and no other symptoms. The diesel exhaust fluid (DEF) had not become contaminated.
Details: When the tech removed the reductant injector, a large blob of carbon was found attached to the end of the injector, which was causing a restricted / poor spray pattern.
Confirmed Repair: The tech cleaned the carbon off the end of the injector and its mounting hole, re-installed the injector, cleared code and placed the vehicle back in service. The code did not return.