SPN 1243 FMI 3BrakesLow
Retarder Enable — Voltage Above Normal
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
DieselVision™ Upload photos — DieselVision™ reads diagrams, connectors, and screens with precision.
Overview
Fault Code
SPN 1243 / FMI 3
Category
Brakes
Fault Type
sensor
FMI Category
voltage high
Severity
Low
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Retarder enable switch — cab, dash or steering column
Common Symptoms
- Retarder enable signal fault
- Retarder not functioning correctly
- Retarder always on or always off
Root Causes
- Retarder enable switch shorted high
- Wiring short to power
- Switch failed
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.
Derate Behavior
Power Derate
No
Mileage / Regen Notes
Switch failure common at 300k–500k miles.
System Interactions
SPN 1243 FMI 3 interacts with the following systems and may trigger cascading faults if not resolved promptly.
TSB Reference: Allison TSB 2019-011 — Retarder Enable Switch 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.
OEM Notes
Allison
Check switch mounting for looseness.
Eaton
Check wiring harness routing.
Meritor
Check connector for corrosion.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 1243 FMI 3 — Full FSM Sequence & Repair Steps — includes wiring notes, sensor specs, and gated repair procedures.
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.
Get weekly diesel diagnostic tips
Real-world fault code walkthroughs, wiring tips, and shop-tested fixes — free.
No spam. Unsubscribe anytime.
Get AI-Guided Diagnosis for SPN 1243 FMI 3
The diagnostic engine walks you through every physical test — step by step, on your phone, at the truck. No guessing. No wasted parts.