Diagnostic Overview
Cummins fault code 2771 is commonly associated with SCR aftertreatment performance, DEF dosing, or NOx reduction issues. The ECM may detect that the SCR system is not reducing NOx emissions as expected. Causes may include poor DEF quality, contaminated DEF, a blocked DEF doser, low DEF pressure, faulty NOx sensors, exhaust leaks, crystallization in the DEF system, or an SCR catalyst that is no longer efficient. Diagnosis should include DEF quality testing, DEF pressure verification, doser inspection, NOx sensor data comparison, and checking for exhaust leaks before replacing major components.