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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)
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Questions?
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Call our service desk or send a request. We're happy to talk through any of these topics.