Diagnostic Overview
Cummins fault code 1977 is generally tied to aftertreatment regeneration performance or DPF restriction. The ECM may not be seeing the expected reduction in soot load or may detect that exhaust flow through the DPF is outside the normal range. Common causes include incomplete regenerations, excessive idle time, a plugged DPF, faulty DPF differential pressure sensor, failed exhaust temperature sensor, exhaust leaks, or upstream engine problems creating too much soot. Diagnosis should focus on DPF pressure readings, exhaust temperature readings, regen history, and whether the filter has reached ash capacity.