Diagnostic Overview
SPN 4374 is commonly associated with the DEF/SCR dosing system or aftertreatment fluid control. In a derate situation, it may indicate that the DEF system cannot dose properly or is not meeting commanded pressure or flow requirements. On PACCAR MX engines, confirm the exact definition with DAVIE4, the engine serial number, emissions year, and current calibration because code meanings and troubleshooting trees can vary.
Likely Causes
Low DEF; poor DEF quality; clogged DEF filter; crystallized DEF doser; failed DEF pump; restricted DEF line; wiring issue; DEF pressure sensor fault.
What To Check First
Test DEF quality, inspect for crystallization, verify DEF pump pressure and line condition, perform a dosing test, and check for related SCR efficiency or NOx sensor faults.