SPN 4364 FMI 18 — SCR Conversion Efficiency — Below Threshold

What SPN 4364 FMI 18 means

The SCR catalyst isn't reducing NOx as much as it should — measured conversion efficiency is below the required threshold. The ECM flags it and, if it persists, stages a derate. The cause is usually upstream of the catalyst (DEF dosing/quality or sensors), not the catalyst itself.

How serious is it?

Serious. Power derate likely — plan to stop soon.

Most likely causes

1. Insufficient or off-spec DEF dosing (clogged injector, poor DEF quality)
2. Inlet/outlet NOx sensor drift or failure
3. DEF dosing/supply pressure fault starving the SCR
4. Aged, contaminated, or (rarely) tampered SCR catalyst

First moves (roadside)

1. Verify DEF quality and level first — bad/contaminated DEF is the #1 cause of low SCR efficiency.
2. Check for companion dosing (SPN 3361) and NOx-sensor codes that explain the low conversion.
3. Inspect the dosing injector and lines for crystallization or restriction.
4. Confirm both NOx sensors are reporting plausible values before condemning the catalyst.

Common misdiagnoses

• Replacing the expensive SCR catalyst when the real problem was DEF quality or a clogged doser.
• Swapping one NOx sensor without confirming which one drifted.
• Clearing the code without fixing dosing — efficiency stays low and it returns.

FAQ

Related codes

Codes that commonly appear alongside SPN 4364 FMI 18, or share the same subsystem.

• Aftertreatment 1 Diesel Exhaust Fluid Quality - Data Valid But Below Normal Operating Range, Most Severe Level (DEF quality/concentration incorrect) (SPN 4364 FMI 1)
• 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)
• DPF Differential Pressure — Data Erratic (SPN 3251 FMI 2)
• DEF Dosing Unit — Not Responding Properly (SPN 3361 FMI 7)
• DEF Quality — Out of Spec (SPN 4094 FMI 31)
• EGR Temperature — Too High (SPN 412 FMI 0)
• SCR NOx Catalyst Efficiency Below Threshold — Bank 1 (P20EE)

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

All Aftertreatment & Emissions codes

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