Diagnostic Overview
Cummins fault code 2777 usually points to a DEF dosing or SCR system performance concern. The ECM may command DEF injection but not see the expected emissions response, or it may detect a dosing system fault. Common causes include crystallized DEF at the doser, contaminated DEF, a clogged DEF filter, a weak DEF pump, wiring faults, failed NOx sensors, or SCR catalyst issues. A technician should inspect the DEF doser, verify DEF quality and pressure, compare inlet and outlet NOx readings, and correct any related DEF or sensor codes.