Diagnostic Overview
Cummins fault code 4277 is commonly associated with SCR/DEF system performance or emissions inducement status. The ECM may be limiting operation because the aftertreatment system has not met required emissions targets. Possible root causes include contaminated DEF, failed DEF doser, DEF pump faults, faulty NOx sensors, exhaust leaks, wiring problems, or SCR catalyst failure. This code should be treated as part of a larger aftertreatment diagnosis, with the root cause identified from the full active fault-code list.