Detroit Diesel Fault Code
Free · no signupSPN 4364 FMI 18
DEF Light Active / Heavy Derate
System
Emissions
Symptom
DEF Light Active / Heavy Derate
Diagnosis & fix steps
LOGIC: Selective Catalytic Reduction (SCR) NOx Conversion Efficiency Low.
ACTION: Test DEF quality with a refractometer (32.5% spec). Inspect DEF dosing nozzle tip for heavy urea buildup crust blockages.
Red Seal field insights
Conversion Math: Trailing downstream NOx sensor data remains too close to inlet NOx logs, indicating the catalyst is failing to split emissions.
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