What are the symptoms of SPN 3480 FMI 0?

SCR conversion efficiency critically low - NOx emissions far above threshold. Severe engine derate active - typically 5 mph or idle-only mode. Inducement active or final inducement stage - vehicle may not be driveable. Multiple aftertreatment fault codes active alongside SPN 3480. DEF system may appear to be functioning but SCR catalyst is not converting NOx. Possible companion faults: SPN 4364/18 (SCR efficiency moderately low), SPN 5246/31 (inducement active)

What causes SPN 3480 FMI 0?

SCR catalyst severely degraded or poisoned - catalyst completely ineffective. DEF dosing system complete failure - no DEF being delivered to SCR. DEF quality severely out of specification - wrong fluid in DEF tank (diesel, water, etc.). SCR catalyst temperature chronically too low - catalyst never reaching operating temperature. Upstream and downstream NOx sensor both failed - false efficiency calculation. Accumulated unanswered inducement warnings - SPN 4364 ignored until reaching critical threshold

How do I diagnose SPN 3480 FMI 0?

Full step-by-step DVOM tests, electrical procedures, and connector pinout data for SPN 3480 FMI 0 are available inside the AI Diesel Tech guided diagnostic session. The AI walks you through every test — one step at a time.

SPN 3480 FMI 0AftertreatmentCriticalInducement Risk

Aftertreatment 1 SCR Conversion Efficiency - Data Valid But Above Normal (Critical - Inducement Imminent)

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Overview

Fault Code

SPN 3480 / FMI 0

Common Symptoms

SCR conversion efficiency critically low - NOx emissions far above threshold
Severe engine derate active - typically 5 mph or idle-only mode
Inducement active or final inducement stage - vehicle may not be driveable
Multiple aftertreatment fault codes active alongside SPN 3480
DEF system may appear to be functioning but SCR catalyst is not converting NOx
Possible companion faults: SPN 4364/18 (SCR efficiency moderately low), SPN 5246/31 (inducement active)

Root Causes

SCR catalyst severely degraded or poisoned - catalyst completely ineffective
DEF dosing system complete failure - no DEF being delivered to SCR
DEF quality severely out of specification - wrong fluid in DEF tank (diesel, water, etc.)
SCR catalyst temperature chronically too low - catalyst never reaching operating temperature
Upstream and downstream NOx sensor both failed - false efficiency calculation
Accumulated unanswered inducement warnings - SPN 4364 ignored until reaching critical threshold

Full step-by-step tests, DVOM guidance, and pinout data are available inside the AI Diesel Tech diagnostic session.

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Derate Behavior

Power Derate

Yes

SCR Inducement Risk

If left unresolved, this fault may trigger SCR inducement — a progressive derate enforced by the ECM to ensure emissions compliance.

Mileage / Regen Notes

SCR catalyst degradation increases with mileage - critical failure typically at 500k+ miles without maintenance. DEF quality issues can cause critical efficiency failure at any mileage.

System Interactions

SPN 3480 FMI 0 interacts with the following systems and may trigger cascading faults if not resolved promptly.

DPF differential pressure monitoring — soot load calculations affected

SCR NOx conversion efficiency — DEF dosing strategy may be altered

Active regen inhibit logic — regen may be blocked or delayed

Engine protection derate — ECM may reduce power to protect components

TSB Reference: Cummins TSB 4021912 - SCR Critical Efficiency Fault and Inducement Reset; Detroit TSB 18-012 - SCR Conversion Efficiency Critical Diagnosis; PACCAR TSB AT-002 - SCR Critical Efficiency Diagnosis

Common Diagnostic Mistakes

Replacing the sensor without verifying wiring continuity and supply voltage first.

Clearing codes without identifying and resolving the root cause — the fault will return.

Ignoring related active faults — cascading faults often share a root cause.

Skipping the connector inspection — corrosion and damaged pins are a leading cause of sensor faults.

Condemning the ECM before exhausting all wiring and component tests.

Replacing the DPF without verifying the regen system is functional — a failed regen injector will reload the new DPF.

Ignoring DEF quality — contaminated DEF causes SCR efficiency faults that look like hardware failures.

OEM Notes

Cummins

X15: SPN 3480 requires INSITE for inducement reset. SCR catalyst replacement is common when this fault is present without a repairable root cause. Catalyst replacement requires INSITE recalibration.

Detroit

DD15: DDDL catalyst desulfation procedure should be attempted before condemning catalyst. If desulfation does not improve efficiency, catalyst replacement required.

PACCAR

MX-13: Wrong fluid in DEF tank is a common cause of SPN 3480 on MX-13. Drain and flush entire DEF system including lines and doser before replacing catalyst.

Volvo

MP8: SCR catalyst replacement on MP8 requires PTT recalibration. Verify DEF quality and dosing before condemning catalyst - catalyst replacement is expensive.