NIT Excavator Operator + Diagnostic Guide G4 operator action · SPN/FMI · technician direction
Excavator Fault Codes
SPN/FMI &
Monitor Messages
operator action → SPN/FMI → status → occurrence count → technician diagnosis
A two-level guide for plant operators, technicians and advanced learners: what to do when a fault code appears, how to read SPN/FMI, status and occurrence count, and how that evidence guides diagnosis.
Read the complete diagnostic message.
How should a plant operator and technician read the same fault code?
Plant operator: read and record the complete code, monitor message, machine hours and what the excavator was doing when the code appeared. Technician: use the SPN, FMI, code status and occurrence count to select the correct system or circuit for testing. In a J1939 DTC, the SPN identifies what is being reported, the FMI identifies how it is abnormal, and the Occurrence Count (OC) can show how many times the condition has activated. [1]
Plant operator first
What to do when a fault code appears in the cab
The operator’s first job is to make the fault information useful. Control the machine, read the whole monitor, record the evidence and hand over a clear report. The deeper circuit or system testing comes after that evidence is captured.
Control
Keep the excavator stable and follow the operating instruction shown on the monitor.
Capture
Record the full code, SPN/FMI where shown, status, occurrence count and exact monitor message.
Context
Record machine hours, the work being done and any change in gauges, engine response or hydraulics.
Report
Pass the evidence to the responsible instructor, supervisor or technician so diagnosis starts from accurate information.
The plant operator captures what the machine displayed and how it behaved. The technician or advanced learner uses that evidence to choose the circuit, controller network or machine system to test.
Code structure
SPN tells you what. FMI tells you how.
A diagnostic code becomes useful when its parts are read together. The SPN identifies the subject of the diagnostic report. The FMI describes the failure mode affecting that subject.
Suspect Parameter Number
What is being reported?
The parameter or function may relate to temperature, pressure, speed, voltage, a sensor, an actuator, aftertreatment or controller communication.
Failure Mode Identifier
How is it abnormal?
The failure mode can describe a value above or below normal, an electrical circuit condition, an abnormal mechanical response, a communication problem or another defined state.
SPN identifies the parameter. FMI identifies the condition. The diagnostic direction comes from the combination.
Plant operator view
Know what the monitor is reporting
Use the SPN/FMI pair to record the fault accurately, connect it with the monitor message and describe what changed in the machine.
Technician / advanced learner view
Use the pair to choose the diagnostic path
The SPN/FMI combination directs testing toward a measured condition, electrical circuit, mechanical response, communication path or another defined diagnostic state.
NIT practical demonstration 1
Same SPN, different FMI, different diagnostic direction
During excavator diagnostic instruction at Newton Institute of Technology, the learner starts at the monitor and reads the complete code before moving to the machine.
SPN 110 FMI 0
- SPN 110: Engine coolant temperature.
- FMI 0: Data valid but above normal operating range — most severe level.
- Plant operator: Record the code, monitor message, temperature indication, machine hours and operating condition.
- Technical direction: An actual high coolant-temperature condition. An official J1939 display example shows “Engine Coolant Temperature Above Normal” with SPN 110 FMI 0. [2]
SPN 110 FMI 4
- SPN 110: The parameter is still engine coolant temperature.
- FMI 4: Voltage below normal or shorted to a low source.
- Plant operator: Record the full code and whether the temperature gauge and machine behaviour appear normal or changed.
- Technical direction: The focus moves from a temperature condition to the electrical signal/circuit. The complete SPN/FMI pair determines what is inspected next. [3]
Topic boundary: Article 184 interprets the fault code. For radiator airflow, coolant circulation and overheating inspection, continue to NIT’s Excavator Cooling System guide. [6]
Reference table
FMI meanings used in J1939 diagnostic codes
The FMI gives the failure mode associated with the SPN. The table below follows SAE J1939 diagnostic terminology. [3]
| FMI | Meaning | Diagnostic direction |
|---|---|---|
| 0 | Data valid but above normal operating range — most severe | Parameter genuinely high at the highest defined severity |
| 1 | Data valid but below normal operating range — most severe | Parameter genuinely low at the highest defined severity |
| 2 | Data erratic, intermittent or incorrect | Signal quality, continuity or rationality |
| 3 | Voltage above normal or shorted high | High-voltage electrical path |
| 4 | Voltage below normal or shorted low | Low-voltage electrical path |
| 5 | Current below normal or open circuit | Open/low-current circuit |
| 6 | Current above normal or grounded circuit | High-current/grounded circuit |
| 7 | Mechanical system not responding properly or out of adjustment | Command versus physical response |
| 8 | Abnormal frequency, pulse width or period | Frequency/timing signal |
| 9 | Abnormal update rate | Network/controller communication |
| 10 | Abnormal rate of change | Parameter changing at an implausible rate |
| 11 | Root cause not known | Further diagnosis required |
| 12 | Bad intelligent device or component | Controller/intelligent device condition |
| 13 | Out of calibration | Calibration or learned-value issue |
| 14 | Special instructions | Follow the defined diagnostic instruction |
| 15 | Data valid but above normal — least severe | High parameter, lower severity |
| 16 | Data valid but above normal — moderately severe | High parameter, moderate severity |
| 17 | Data valid but below normal — least severe | Low parameter, lower severity |
| 18 | Data valid but below normal — moderately severe | Low parameter, moderate severity |
| 19 | Received network data in error | Network data validity |
| 20 | Data drifted high | Signal remains plausible but has shifted high |
| 21 | Data drifted low | Signal remains plausible but has shifted low |
| 22–30 | Reserved for SAE assignment | Use current machine/service documentation |
| 31 | Condition exists | SPN identifies the condition that is present |
On a small screen, swipe horizontally to view the full table.
Two-level FMI reading
What the FMI means to the operator — and where it sends the technician
The operator needs a clear practical meaning. The technician needs the deeper diagnostic direction. Read the same FMI at the level that matches your role.
| FMI family | Plant operator reads it as | Technician / advanced learner direction |
|---|---|---|
| 0 / 15 / 16 | The reported parameter is high. Record the FMI, monitor message, gauge reading and machine behaviour. | Confirm the actual high condition and interpret the defined severity level. |
| 1 / 17 / 18 | The reported parameter is low. Record the code and any related change in the machine. | Confirm the actual low condition and interpret the defined severity level. |
| 2 | The signal is erratic, intermittent or incorrect. Note when the code appears — for example during vibration, heat or movement. | Test signal quality, continuity and rationality against related values. |
| 3 / 4 | An electrical voltage condition has been detected. Capture the complete code and related monitor message. | Test the signal, supply/reference, ground, connector, harness and controller input as applicable. |
| 5 / 6 | An electrical current/open/grounded condition has been detected. Record the full DTC and operating context. | Test the controlled circuit for open, low-current, high-current or grounded conditions. |
| 7 | A commanded machine function is not responding as expected. Record which function feels slow, weak, delayed or abnormal. | Compare controller command, actuator operation, hydraulic/mechanical response and feedback. |
| 9 / 19 | The controllers or network data are not communicating normally. Record all codes or linked monitor messages shown together. | Check controller power/ground, data-link wiring, connectors, termination and network data validity. |
| 20 / 21 | A signal has drifted high or low. Record the operating condition when the code appears. | Evaluate sensor rationality, calibration and comparison with related measurements. |
| 31 | A defined condition exists. Use the SPN to identify which condition the machine is reporting. | Follow the SPN-specific diagnostic procedure for that machine and controller. |
On a small screen, swipe horizontally to compare the operator and technician columns.
NIT practical demonstration 2
FMI 4: operator evidence first, technician circuit test second
When the monitor identifies an FMI 4 condition, the plant operator and technician use the same code differently — but the two roles connect directly.
Plant operator
Capture the fault in context
Record the SPN/FMI, active or stored status, occurrence count, machine hours, exact monitor message and what the excavator was doing when the fault appeared.
Technician / advanced learner
Trace the low-voltage path
FMI 4 directs the diagnostic work toward the device and its electrical path until testing identifies the actual cause.
Parameter
Identify the SPN and the parameter being reported.
Device
Identify the sensor or monitored component associated with that parameter.
Circuit
Trace the connector, harness, supply or reference, ground and controller path.
Test
Measure the relevant circuit and confirm the cause before selecting the repair.
For the complete battery, starter, alternator, wiring, sensor and controller relationships, use NIT’s Excavator Electrical System guide. [5]
Diagnostic status
Active and previously active fault codes
The status tells you whether the controller is detecting the condition now or whether you are reading diagnostic history.
Current condition
Active fault
The electronic controller is detecting the condition now. The DTC represents the machine’s current diagnostic state.
Diagnostic history
Previously active / stored fault
The condition was detected earlier and the record remains available for diagnosis. This is especially useful with intermittent faults that are not active during inspection.
Record whether the code is active now or appears in stored history, and note whether the machine symptom is present at the same time.
Technician / advanced learner: Active status helps test a current condition. Stored history helps trace faults that appear intermittently and may not be present during inspection.
SAE J1939-73 includes services for broadcasting active DTCs and reading or clearing diagnostic information. [1] An official fault-history display example also shows stored fault information together with the engine hours at which it occurred. [2]
Fault history
What does occurrence count mean?
In a J1939 diagnostic trouble code, the Occurrence Count (OC) records how many separate times the fault has been detected as active. The DTC data structure allocates seven bits to occurrence count. [3]
The parameter and failure mode are the same as SPN 110 FMI 4, while the occurrence count shows that the condition has become active repeatedly.
Plant operator
Record the OC exactly
Occurrence count tells the handover team whether the machine has logged the condition once or many times.
Technician / advanced learner
Use recurrence as diagnostic evidence
A repeated count can support investigation of intermittent wiring, vibration-related faults, temperature-sensitive faults or recurring communication problems.
Monitor formats
J1939 codes and manufacturer-specific monitor codes
Excavators can present diagnostic information as SPN/FMI combinations, manufacturer-specific diagnostic numbers, event codes, controller-specific codes, written monitor messages, or a combination of these.
Plant operator
Record exactly what the machine shows
Capture the make/model, complete code, monitor wording, status and operating condition without translating the code from memory.
Technician / advanced learner
Use the correct machine-specific procedure
Match the exact machine, controller and displayed code with the appropriate service information or diagnostic tool before testing.
For detailed dashboard-symbol meanings and warning urgency, use NIT’s Excavator Dashboard Warning Lights & Symbols guide. [4]
Operator-to-technician bridge
How operator evidence becomes technician diagnosis
Operator sees
Read the complete DTC and monitor message exactly as displayed.
Operator describes
Add machine hours, operating condition, status, occurrence count and what changed in the excavator.
Technician interprets
Use SPN/FMI, status and history to select the relevant circuit, network or machine system.
Technician tests
Measure and inspect the selected system until the root cause is confirmed and the correct repair is identified.
The quality of the diagnosis improves when the operator hands over complete evidence instead of only saying that a warning appeared.
Clear role split
Where the plant operator role ends and the technician role begins
Plant operator
Observe, capture and communicate
Read the monitor, record the code, identify when it appeared, describe machine behaviour and follow the operating instruction shown by the machine.
Technician / advanced learner
Interpret, test and confirm
Use service information, diagnostic tools and physical testing to trace the relevant circuit or system, confirm the root cause and select the repair.
NIT operator method
Use the NIT CODE method when a fault appears
Record SPN, FMI, occurrence count, status and the exact monitor message.
Record whether the machine was starting, digging, loading, travelling, lifting or idling.
Record what changed in engine response, hydraulics, gauges, noise, smoke, vibration or temperature.
Pass the complete evidence to the responsible instructor, supervisor or technician.
NIT practical demonstration 3
What a useful excavator fault report looks like
During practical training, learners report the evidence in a form that another person can act on.
| Machine hours | 4,286.7 |
|---|---|
| Operation | Loading material |
| Code | SPN 110 FMI 4 |
| Status | Active |
| Occurrence count | 6 |
| Engine response | Normal |
| Hydraulic response | Normal |
| Monitor | Engine-system diagnostic message displayed |
Operator value: the report captures what happened. Technician value: it provides the evidence needed to begin targeted testing instead of starting from a vague complaint.
Operator checklist
Record these details with every significant code
| Information | What to record |
|---|---|
| Machine | Manufacturer and model |
| Hours | Exact hour-meter reading |
| SPN / code | Complete number as displayed |
| FMI | Complete failure-mode number |
| Occurrence count | OC value, when displayed |
| Status | Active or previously active/stored |
| Monitor message | Exact wording |
| Operation | What the excavator was doing |
| Machine response | Engine, hydraulics, gauges and other changes |
Quick reference
Excavator fault-code FAQs
What should a plant operator do first when a fault code appears?
Read the complete monitor, record the exact code and message, note machine hours and operating conditions, and report the evidence clearly. The technician can then use that information for targeted diagnosis.
What part of fault-code diagnosis belongs to the technician?
The technician interprets the code against the correct machine information, selects the relevant circuit or system, performs measurements and inspections, confirms the root cause and determines the repair.
What does SPN mean on an excavator?
SPN means Suspect Parameter Number. It identifies the parameter or function associated with the diagnostic information.
What does FMI mean on an excavator?
FMI means Failure Mode Identifier. It describes the type of abnormal condition associated with the SPN.
What does OC mean in a fault code?
OC means Occurrence Count. It records how many separate times the diagnostic condition has become active. [3]
What is the difference between an active and stored fault?
An active fault is being detected now. A stored or previously active fault was detected earlier and remains available in diagnostic history.
What does FMI 4 mean?
FMI 4 identifies voltage below normal or a short toward a low source. The relevant electrical path is then tested to confirm the cause.
What does FMI 7 mean?
FMI 7 identifies a mechanical system that is not responding properly or is out of adjustment.
What does FMI 9 mean?
FMI 9 identifies an abnormal update rate and commonly directs diagnosis toward controller or network communication.
Are all excavator fault codes SPN/FMI codes?
No. Excavators can also use manufacturer-specific diagnostic numbers, event codes and written monitor messages.
Does a fault code identify the exact repair?
A fault code identifies the detected condition and establishes the diagnostic direction. Testing confirms the root cause and the correct repair.
Two-role memory cue
Operator captures the evidence. Technician confirms the cause.
Operator: code · message · status · OC · machine hours · operating condition · machine behaviour. Technician: SPN/FMI interpretation · system selection · measurement · root-cause confirmation · repair.
Learn the machine beyond the controls
Register for Excavator Operator Training at Newton Institute of Technology
Build practical excavator skills in machine familiarisation, controls, digging, loading, travelling, monitor interpretation, warning recognition, daily checks and clear fault reporting — with enough diagnostic awareness to communicate effectively with maintenance personnel.
CourseExcavator Operator Training
Duration2 months · 70% practical / 30% theory
IntakeMonthly · Migori Town
Technical references
- SAE International. SAE J1939-73_202609 — Application Layer — Diagnostics. Revised 9 September 2026. Defines J1939 diagnostic services including active DTC broadcasts, diagnostic-lamp status and reading/clearing diagnostic information. SAE Mobilus.
- Cummins Inc. Official operator/service manual — J1939 fault display and fault history examples. Includes a FAULT INFO example displaying “Engine Coolant Temperature Above Normal” with SPN 110 FMI 0 and a fault-history screen showing stored fault information. Cummins technical manual.
- SAE technical reference transmitted to UNECE WP.29. SAE J1939-73 diagnostic message structure. Defines SPN/FMI encoding, occurrence count and FMI terminology, including FMI 20/21 data-drift meanings. UNECE-hosted technical document.
- Newton Institute of Technology. Excavator Warning Lights & Dashboard Symbols: Meanings. Dedicated guide to warning symbols, monitor warnings and first operator response. niteducation.com.
- Newton Institute of Technology. Excavator Electrical System: Battery, Starter & Alternator. Dedicated guide to electrical circuits, wiring, sensors, controllers and monitor information. niteducation.com.
- Newton Institute of Technology. Excavator Cooling System: Parts, Overheating & Daily Checks. Dedicated guide to coolant flow, radiator performance and overheating inspection. niteducation.com.
- Newton Institute of Technology. Excavator Operator Training in Kenya. Current NIT course information for the two-month, 70% practical training programme and monthly intake. niteducation.com.
Prepared by: Newton Institute of Technology Editorial Team · Institutional / Technical review: Engineer Simon Barongo · Last reviewed: 20 October 2026.