What are the symptoms of SPN 628 FMI 2?

Intermittent loss of gauge cluster functionality. Multiple unrelated fault codes appearing simultaneously. Erratic speedometer or tachometer readings. Intermittent loss of communication with modules. Cruise control dropping out randomly. ABS or transmission warning lights intermittent. Engine protection warnings without clear cause

What causes SPN 628 FMI 2?

Damaged, corroded, or loose CAN bus connector pins at ECM or node connections. Missing or incorrect CAN bus termination resistors (should be 60 ohms across CAN Hi/Lo with network powered down). Chafed or shorted CAN Hi/Lo wiring causing intermittent signal dropout. Multiple modules sharing CAN network with one node intermittently pulling bus low. Water intrusion into CAN network connectors or junction boxes. Improper CAN wire shielding or routing near high-EMI sources (alternators, fuel injector wiring). Faulty or failing ECM with degraded internal CAN transceiver. Aftermarket accessories improperly spliced into CAN network. Loose or corroded chassis ground causing common-mode noise on CAN lines

How do I diagnose SPN 628 FMI 2?

Full step-by-step DVOM tests, electrical procedures, and connector pinout data for SPN 628 FMI 2 are available inside the AI Diesel Tech guided diagnostic session. The AI walks you through every test — one step at a time.

SPN 628 FMI 2Network

CAN Bus - Erratic/Intermittent

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Overview

Fault Code

SPN 628 / FMI 2

Common Symptoms

Intermittent loss of gauge cluster functionality
Multiple unrelated fault codes appearing simultaneously
Erratic speedometer or tachometer readings
Intermittent loss of communication with modules
Cruise control dropping out randomly
ABS or transmission warning lights intermittent
Engine protection warnings without clear cause

Root Causes

Damaged, corroded, or loose CAN bus connector pins at ECM or node connections
Missing or incorrect CAN bus termination resistors (should be 60 ohms across CAN Hi/Lo with network powered down)
Chafed or shorted CAN Hi/Lo wiring causing intermittent signal dropout
Multiple modules sharing CAN network with one node intermittently pulling bus low
Water intrusion into CAN network connectors or junction boxes
Improper CAN wire shielding or routing near high-EMI sources (alternators, fuel injector wiring)
Faulty or failing ECM with degraded internal CAN transceiver
Aftermarket accessories improperly spliced into CAN network
Loose or corroded chassis ground causing common-mode noise on CAN lines

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

AI walks you through every test — one step at a time. Plans start at $12.95/mo.

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Derate Behavior

Power Derate

Yes

Mileage / Regen Notes

SPN 628 FMI 2 (CAN Bus Erratic/Intermittent) can inhibit active DPF regeneration if the aftertreatment controller or DCU loses reliable communication with the ECM over J1939. Incomplete or interrupted regen commands may result in parked regen lockout and illumination of high soot load warnings. Some platforms will derate engine power and disable active regen until network integrity is restored.

System Interactions

SPN 628 FMI 2 interacts with the following systems and may trigger cascading faults if not resolved promptly.

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.

Related Faults in This Category

SPN 5246-31SCR NOx Efficiency Low SPN 4364-18DEF Dosing Below Normal SPN 3251-0DPF Differential Pressure High SPN 157-18Fuel Rail Pressure Low

OEM System Resources

Cummins Fault Codes Detroit Diesel Fault Codes PACCAR Fault Codes Volvo Fault Codes Freightliner Fault Codes Mack Fault Codes

ASE Training Resources

Why ASE Certification Matters How ASE Tests Work ASE Study Tips ASE vs AI Diesel Tech Certification Paths Get Started with ASE

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Authority References

Diagnostic procedures on this page are informed by OEM service literature and industry standards.

Cummins QuickServe OnlineOfficial Cummins service manuals, fault code lookup, and TSB database.Detroit Diesel DiagnoCentralDetroit Diesel diagnostic procedures, DDDL documentation, and service literature.PACCAR MX Engine ServicePACCAR MX-11 and MX-13 engine service documentation and DAVIE diagnostic guides.Volvo/Mack Tech ToolVolvo and Mack engine fault code reference and PTT/Tech Tool diagnostic procedures.ASE — Diesel Truck Technician CertificationNational Institute for Automotive Service Excellence — T2 Diesel Engines certification standards.EPA Aftertreatment & Emissions GuidelinesU.S. EPA heavy-duty diesel aftertreatment compliance, DPF/SCR regulations, and NOx standards.

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