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DPF & DEF Diagnostic Guide — understanding your emissions system.

Emissions-related faults are among the most common reasons heavy-duty trucks end up in the shop, and the most misunderstood by operators. This guide explains what the fault codes mean.

DPF (Diesel Particulate Filter) basics

The DPF captures soot from exhaust and burns it off during regeneration
Active regen is triggered automatically by the ECM when soot load reaches a threshold
If regen is interrupted repeatedly (short trips, city driving), soot and ash build up
Common fault codes: SPN 3720 (regen required), SPN 3481 (soot load high), SPN 3719 (regen inhibited)
A plugged DPF causes high exhaust temperatures, reduced power, and increased fuel consumption

DEF (Diesel Exhaust Fluid) and SCR system

DEF is injected into the exhaust stream to convert NOx into nitrogen and water
Quality matters: off-spec DEF (low urea concentration) will trigger faults
Common fault codes: SPN 4334 (DEF quality), SPN 5246 (NOx sensor), SPN 3363 (dosing valve)
SCR efficiency faults often mean the NOx sensor or dosing valve needs replacement
DEF tank heaters and lines must function in cold weather: frozen DEF will derate the engine

When to call a technician

Derate or limp mode that doesn't clear after a parked regen
Recurring DPF faults after cleaning (suggests an underlying engine issue)
Multiple active fault codes across different modules (suggests wiring or ECM issue)
Exhaust temperature readings that don't match operating conditions
Any fault that returns immediately after clearing (the problem hasn't been fixed)

Questions?

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