Many modern tractors, engines, loaders, telehandlers and excavators use SAE J1939 diagnostic messages. These often appear as an SPN number together with an FMI number. Understanding the two parts helps narrow down the fault before parts are replaced.

1. What Is J1939?

SAE J1939 is a communication standard widely used on heavy-duty vehicles and off-highway machinery.

It allows electronic controllers to exchange information such as:

• Engine speed
• Coolant temperature
• Fuel pressure
• Transmission status
• Hydraulic information
• Fault messages

2. What Is an SPN?

SPN means Suspect Parameter Number.

The SPN identifies the parameter, signal or circuit involved.

Examples might relate to:

• Coolant temperature
• Engine oil pressure
• Fuel rail pressure
• Boost pressure
• Throttle position
• Battery voltage

3. What Is an FMI?

FMI means Failure Mode Identifier.

The FMI describes how the controller believes the signal or system has failed.

For example, the same SPN may be accompanied by different FMIs depending on whether the signal is too high, too low, intermittent or missing.

4. Why SPN and FMI Must Be Read Together

An SPN by itself is incomplete.

For example, the same pressure sensor SPN could appear with:

• Voltage too high
• Voltage too low
• Signal erratic
• Mechanical response incorrect

The FMI helps distinguish these possibilities.

5. Common FMI Meanings

FMI	General meaning
0	Data valid but above normal operating range – most severe.
1	Data valid but below normal operating range – most severe.
2	Data erratic, intermittent or incorrect.
3	Voltage above normal or shorted high.
4	Voltage below normal or shorted low.
5	Current below normal or open circuit.
6	Current above normal or grounded circuit.
7	Mechanical system not responding correctly.
8	Abnormal frequency, pulse width or period.
9	Abnormal update rate / communication problem.
10	Abnormal rate of change.
11	Failure mode not otherwise identifiable.
12	Bad intelligent device or component.
13	Out of calibration.
14	Special instruction.
15	Data valid but above normal operating range – least severe.
16	Data valid but above normal operating range – moderately severe.
17	Data valid but below normal operating range – least severe.
18	Data valid but below normal operating range – moderately severe.
19	Received network data in error.
31	Condition exists.

Note: these are general SAE descriptions. Manufacturer diagnostic manuals may add further interpretation.

6. FMI 3 – Voltage Above Normal

Common causes include:

• Signal wire shorted to supply voltage
• Open sensor ground
• Failed sensor
• Connector fault
• Incorrect wiring repair

7. FMI 4 – Voltage Below Normal

Possible causes include:

• Signal shorted to ground
• Low reference voltage
• Sensor failure
• Corroded connector
• Harness damage

8. FMI 5 – Current Below Normal / Open Circuit

This often points toward:

• Broken wire
• Disconnected plug
• Open solenoid coil
• Loose terminal
• Corrosion

9. FMI 6 – Current Above Normal

Possible causes include:

• Shorted actuator coil
• Wire shorted to ground
• Internal component fault

10. FMI 2 – Erratic or Intermittent Signal

This is especially common with:

• Loose connectors
• Chafed harnesses
• Vibration-related faults
• Sensor dropout
• Unstable power or ground

11. FMI 7 – Mechanical System Not Responding Properly

FMI 7 can indicate that an actuator is being commanded but the expected mechanical response is not happening.

Possible causes

• Stuck valve
• Seized linkage
• Actuator jammed
• Hydraulic pressure problem
• Mechanical wear

12. FMI 9 – Abnormal Update Rate

This often relates to communication between controllers.

Possible causes:

• CAN wiring fault
• Controller losing power
• Controller offline
• Poor ground
• Network termination problem

13. FMI 13 – Out of Calibration

Common on systems such as:

• Throttle position
• Transmission clutch packs
• Steering angle
• Joystick controls
• Three-point linkage position

A component may be electrically sound but still require calibration.

14. FMI 31 – Condition Exists

This means the controller has detected a defined condition rather than a simple open/short circuit.

Examples can include:

• Regeneration required
• Protection mode active
• Specific operating condition detected

15. What Is an OC or Occurrence Count?

Some displays show how many times the fault has occurred.

A high occurrence count can help identify:

• Long-standing intermittent faults
• Repeated voltage problems
• Recurring connector problems

16. What Is Source Address?

J1939 networks may identify which controller sent the message.

Possible controllers include:

• Engine ECU
• Transmission ECU
• Instrument cluster
• Hydraulic controller
• Aftertreatment controller

The source can be important when the same SPN is used by different controllers.

17. Active vs Previously Active

Active

The controller currently detects the condition.

Previously Active / Stored

The fault occurred earlier but is not currently detected.

Do not automatically ignore stored codes; they can explain intermittent complaints.

18. Multiple SPN/FMI Codes at Once

Several codes can share one root cause.

Check first for

• Low battery voltage
• Charging-system fault
• Controller power supply
• Controller earth
• Shared 5V reference supply
• CAN communication fault

19. Shared 5V Reference Problems

Many sensors share one regulated reference supply.

If one sensor or wire shorts that supply, several unrelated sensor codes may appear at the same time.

20. Low Battery Voltage and J1939 Codes

Low voltage during cranking can cause:

• Communication codes
• Sensor voltage codes
• Controller reset faults
• Multiple stored codes

Always check basic battery and charging condition before chasing numerous electronic faults.

21. CAN High and CAN Low

J1939 communication commonly uses a two-wire CAN network.

Harness faults may include:

• CAN High shorted to ground
• CAN Low shorted to ground
• CAN High shorted to CAN Low
• Open circuit
• Poor termination

22. Connector Inspection

Inspect:

• Water entry
• Green corrosion
• Bent pins
• Pushed-back terminals
• Loose seals
• Broken locks

23. Harness Rub Points

Common trouble areas include:

• Engine mounts
• Cab hinges
• Loader frames
• Hydraulic pipe clamps
• Battery boxes
• Sharp chassis brackets

24. Do Not Replace the ECU First

Before condemning a controller, verify:

• Battery voltage
• Power supplies
• Grounds
• CAN wiring
• Reference voltages
• Sensor and actuator circuits

25. Record Codes Before Clearing Them

Write down:

• SPN
• FMI
• Controller/source
• Occurrence count
• Active or stored status
• Machine operating condition

26. Clearing Codes

Clearing a code only removes the record.

If the fault remains, the code will return.

Clear codes only after:

• Diagnosis
• Repair
• Required calibration
• Verification test

27. When a Code Returns Immediately

An immediate return often suggests:

• Hard electrical fault
• Open circuit
• Short circuit
• Missing power or ground
• Failed component

28. When a Code Returns Only Under Load

Consider:

• Heat-related sensor failure
• Harness movement
• Low system pressure
• Voltage drop
• Mechanical condition outside range

29. Best Diagnostic Order

1. Record the complete code.
2. Confirm active/stored status.
3. Check battery and charging system.
4. Check fuses, powers and earths.
5. Inspect connectors and harness.
6. Follow manufacturer diagnostic steps.
7. Test component only when instructed.
8. Repair and verify.

30. SPN/FMI Quick Reference Checklist

Item	What to record/check
SPN	Exact parameter number.
FMI	Exact failure mode number.
Source	Which controller reported it.
Status	Active or stored.
Occurrence	How often it has happened.
Battery voltage	Check before deep diagnosis.
Power/earth	Confirm controller and sensor supplies.
Harness	Inspect connectors and rub points.
Manual	Use exact diagnostic procedure.

Related Agrimanual Manuals

For a complete repair, use the diagnostic or workshop manual for the exact machine or engine.

• Search Agrimanual for SPN/FMI manuals
• Search Agrimanual manuals containing fault codes
• Search Agrimanual diagnostic manuals
• Request a manual if your machine is not listed

Frequently Asked Questions

What does SPN mean?

SPN means Suspect Parameter Number. It identifies the signal, circuit or operating parameter involved.

What does FMI mean?

FMI means Failure Mode Identifier. It describes the type of fault the controller detected.

Does SPN 123 always mean the same thing on every machine?

The underlying J1939 parameter may be standardised, but the manufacturer’s diagnostic meaning, circuit and repair procedure can still vary.

Why do several SPN/FMI codes appear at once?

Check shared power, ground, 5V reference supplies, battery voltage and CAN communication before replacing several components.

Can I clear SPN/FMI codes without repairing the fault?

You can often clear the record, but if the fault remains the code will usually return.