Failure mode - Cummins 6.7L ISB Diesel (Ram HD, 2019+ generation)

DEF quality / freezing inducements (2019+ 6.7)

Reliability tier: verified - verify before ordering, see /disclosure/

Symptoms

'DEF quality low' warnings and countdown to speed limit
hard derates after cheap/contaminated DEF or a failed DEF heater in freezing weather

Compare with the other causes of the same sign on this engine: loss of power, limp mode or derate (4 causes) · def, dpf and regeneration warnings (2 causes)

Root cause

SCR quality assurance punishes off-spec DEF: the NOx sensors see it and the ECM ramps to inducement. Frozen lines (failed heaters) and diluted DEF mimic sensor faults.

Diagnosis

read DEF quality/NOx DTCs; test DEF with a refractometer before replacing sensors
verify DEF heater operation in freezing climates

Fixes

drain and refill with fresh API-certified DEF; replace failed sensors/heaters as diagnosed ()

Prevalence

occasional

Parts for this repair run about $290-$340: see them next to the other repairs on this engine in repair costs.