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SPN 3224 FMI 18 — SCR NOx conversion efficiency below threshold
CautionAftertreatmentLimp to the next safe stop
Applies to: Sae
SCR NOx conversion efficiency below threshold.
Don't risk it — get a diesel tech on the truck.
Your SPN/FMI rides along, so nearby mechanics see the fault before they quote you.
How serious is it?
Caution. Limp to the next safe stop.
Most likely causes
- Aged SCR catalyst
- Poor DEF quality
- Dosing unit underdosing
- NOx sensor drift
- Temperature sensor drift
First moves (roadside)
- Verify DEF quality (refractometer)
- Verify dosing unit delivers commanded quantity
- Check SCR inlet/outlet temps
- Check NOx sensor accuracy
Common misdiagnoses
- Replacing SCR prematurely
FAQ
What does SPN 3224 FMI 18 mean?
SCR NOx conversion efficiency below threshold.
How serious is SPN 3224 FMI 18?
Caution — Limp to the next safe stop.
What's the most likely cause of SPN 3224 FMI 18?
The most common cause is: Aged SCR catalyst. Other possibilities include Poor DEF quality; Dosing unit underdosing; NOx sensor drift; Temperature sensor drift.
Can I keep driving with SPN 3224 FMI 18?
Limp to the next safe stop. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
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General diagnostic guidance for heavy-duty diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack). Not a substitute for a scan-tool diagnosis on your specific truck.