SPN 3224 FMI 18 — SCR NOx conversion efficiency below threshold
What it means: SCR NOx conversion efficiency below threshold.
How serious / can you keep driving: Caution. Limp to the next safe stop.
Check first: Verify DEF quality (refractometer)
Don't risk it — look up listed providers near you and give them this SPN/FMI when you call.
Listed providers nearest Baltimore, MD
Closest first. Tell them SPN 3224 FMI 18 and what the truck is doing. RoadService.app lists these providers and does not employ them or carry out the repair.
- 0.0 mi away — based in Baltimore, MD · (443) 680-6789
- 0.0 mi away — based in Baltimore, MD · (410) 544-3888
- 24 hour Mobile Diesel Truck Engine Repair DEF Repairs On Site Services Road Service Laptop Diagnostic Tractors & Dump Truck☎ Call0.0 mi away — based in Baltimore, MD · (443) 591-3631
Applies to: Sae
What SPN 3224 FMI 18 means
SCR NOx conversion efficiency below threshold.
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
SCR NOx conversion efficiency below threshold.
Caution — Limp to the next safe stop.
The most common cause is: Aged SCR catalyst. Other possibilities include Poor DEF quality; Dosing unit underdosing; NOx sensor drift; Temperature sensor drift.
Limp to the next safe stop. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Related codes
Codes that commonly appear alongside SPN 3224 FMI 18, or share the same subsystem.
- SCR Operator Inducement — Severe Derate Active (SPN 5246 FMI 0)
- Aftertreatment 1 Diesel Particulate Filter Differential Pressure - Data Valid But Above Normal Operating Range, Most Severe Level (DPF severely restricted / plugged) (SPN 3251 FMI 0)
- Aftertreatment 1 Diesel Exhaust Fluid Quality - Data Valid But Below Normal Operating Range, Most Severe Level (DEF quality/concentration incorrect) (SPN 4364 FMI 1)
- DPF Differential Pressure — Data Erratic (SPN 3251 FMI 2)
- DEF Dosing Unit — Not Responding Properly (SPN 3361 FMI 7)
- SCR Conversion Efficiency — Below Threshold (SPN 4364 FMI 18)
- DEF Quality — Out of Spec (SPN 4094 FMI 31)
- EGR Temperature — Too High (SPN 412 FMI 0)
Same symptom: DEF / SCR fault
Other codes in the DEF and SCR chain — the ones that end in an inducement countdown.
- Reductant System Performance Bank 1 (P204F) · Aftertreatment & Emissions
- Reductant Quality Deviation (P20EA) · Aftertreatment & Emissions
- DEF tank level low (SPN 1761 FMI 18) · Aftertreatment & Emissions
- DEF tank temp sensor — voltage high (SPN 3031 FMI 3) · Aftertreatment & Emissions
- Upstream NOx sensor — internal failure reported (SPN 3216 FMI 12) · Aftertreatment & Emissions
- SCR inlet NOx sensor — erratic (SPN 3216 FMI 2) · Aftertreatment & Emissions
Find a mechanic or related guides
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.