Diagnostic Overview
SPN 5246 FMI 0 is generally an aftertreatment inducement or emissions-system status fault. On a PACCAR MX, this type of code often appears after unresolved DPF, DEF, SCR, NOx sensor, or aftertreatment performance problems have been active long enough to trigger a warning or derate stage. It is usually a result code, not the original failure. 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
Unresolved DEF/SCR fault; DPF restriction; failed regeneration; NOx sensor fault; DEF quality issue; DEF dosing problem; exhaust temperature fault; aftertreatment communication issue; prior codes cleared without correcting the root cause.
What To Check First
Do not diagnose this code alone. Scan the complete code list, repair the root aftertreatment fault, verify DEF quality and dosing, confirm DPF/SCR operation, and complete the required service reset or drive cycle.