Diagnostic Overview
Mack fault code P20EE-00 indicates SCR NOx catalyst efficiency below threshold. The ECM is seeing that NOx reduction across the SCR system is not meeting expected levels. This is a common derate-related aftertreatment code and may be caused by DEF quality, dosing, NOx sensor, exhaust leak, or SCR catalyst problems.
Likely Causes
Poor DEF quality; DEF dosing failure; crystallized doser; faulty NOx sensor; exhaust leak; SCR catalyst contamination or failure.
What To Check First
Check DEF quality first, then run dosing and NOx conversion tests, inspect for exhaust leaks, and compare upstream/downstream NOx readings.