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Cummins Engines · 3868
Aftertreatment DEF Quality Fault
What this code means
Aftertreatment Diesel Exhaust Fluid Quality - Abnormal Update Rate (DEF quality fault; derate after delay period per WholeFleet)
Applies to: all electronic engines.
Possible causes / checks (from the source)
SPN 3364, FMI 9
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Ask for the replacement part on WhatsAppSource: 30-Minute DPF Clean — Cummins fault code list; cross-checked vs WholeFleet Cummins fault code list. Cross-checked in two independent sources. Compiled 2026-10-30. No manufacturer affiliation.