SPN • FMI • Heavy-Duty Diagnostics • Blair NE

Heavy-Duty Truck Fault Code Lookup

Have a fault code but not a confirmed diagnosis? Search the STS starter library for common SPN/FMI combinations and learn what the code generally indicates, what system is involved, and why hands-on testing may still be required.

Search a Fault Code Search by Truck Symptom

Good to Know

  • A fault code is evidence — not always the failed part
  • SPN identifies the parameter or system being reported
  • FMI describes the type of fault being detected
  • STS can test the system and confirm the root cause

What Do SPN and FMI Mean?

Heavy-duty vehicles commonly use SAE J1939 diagnostic messaging. The combination matters: the SPN tells you what parameter or system is being reported, while the FMI describes the failure mode being detected.

SPN

Suspect Parameter Number. It identifies the parameter, signal, component, or system associated with the diagnostic message.

FMI

Failure Mode Identifier. It describes how the parameter is considered abnormal — for example, high, low, implausible, or otherwise outside expected behavior.

OEM Context Still Matters

The same diagnostic framework is used across heavy-duty systems, but troubleshooting steps, calibrations, conditions and repair procedures can be manufacturer-specific.

Do Not Replace a Part Based on the Code Alone

A code tells you what the control system detected. It may be caused by the component named in the code, but it can also involve wiring, connectors, power or ground, leaks, pressure or temperature conditions, communication problems, another active fault, or system performance. Proper testing is what turns a code into a diagnosis.

Where Do I Find My Fault Code?

Your truck may display diagnostic information differently depending on manufacturer, model, and instrument cluster. This temporary visual shows the basic idea: locate the diagnostic display and record the complete SPN and FMI combination whenever possible.

Illustrated guide showing a truck dashboard fault display and an example SPN 4364 FMI 18 fault-code combination
Temporary STS visual guide. Dashboard menus and displays vary by truck and manufacturer.

How This STS Library Will Grow

We are starting small on purpose. STS will add codes as they are verified against authoritative manufacturer or industry information and as we see real diagnostic demand from drivers and fleets.

Detroit / Freightliner

The first public starter entries use Detroit/DTNA diagnostic material that can be verified against published technical documents.

Cummins

Cummins fault-code information is engine-specific. For codes not yet in the STS library, technicians can use the proper Cummins service-information path and engine identification during diagnosis.

J1939

SAE maintains the J1939 framework and Digital Annex. STS will use that framework carefully without presenting this page as a replacement for licensed OEM or SAE service information.

Use the STS Truck Problem Diagnostic Guide to start with what the driver is actually experiencing.

Truck Problem Diagnostic Guide → Diagnostics & Electrical → EGR & DPF Repair → Cummins Repair → Freightliner Repair →

Have the Code? Let STS Find the Cause.

Bring us the complete fault code, truck information, engine information, and symptoms. We can use that information as the starting point for real diagnostics.

Call 402-533-2056 Request Diagnostics