Running off-road machinery efficiently means understanding the language your equipment speaks when something goes wrong. When a diagnostic problem appears, your machine doesn’t just “fail”—it produces a data message known as an SPN FMI code. If you’ve ever stared at codes like 639:9 or 4364-1, this guide will help you decode every part of it, learn what it means, and how to respond before downtime becomes expensive.
What Are SPN and FMI Codes?
Every modern piece of heavy equipment generates diagnostic data through its Electronic Control Module (ECM). The J1939 communication standard translates this data into codes that operators or mechanics can read.
- SPN (Suspect Parameter Number): Identifies which parameter, component, or sensor is at fault. Each number corresponds to a specific part, such as engine oil pressure, fuel temperature, or aftertreatment systems.
- FMI (Failure Mode Identifier): Explains how the component failed—it could be electrical, mechanical, or related to abnormal signals.
Together, an SPN FMI code allows operators to isolate the problem quickly without disassembling machinery entirely.
SPN: FMI Fault Codes List
With thousands of code combinations, a full SPN FMI code list may look overwhelming, but each belongs to a broader diagnostic family based on the system affected. Below is a comprehensive summary covering all SPN and FMI combinations commonly found in Caterpillar off-road equipment—derived from decades of field diagnostics and factory data.
1. Engine and Power System Faults
| SPN | FMI | Description | Severity |
|---|---|---|---|
| 100 | 4 | Engine Oil Pressure – Voltage Below Normal or Shorted to Low | Severe |
| 101 | 0 / 2 / 4 / 16 | Crankcase Pressure – Above Normal, Intermittent, Below Voltage or Moderately Severe | Moderate–Severe |
| 108 | 3 / 13 | Barometric Pressure – Voltage Above Normal / Out of Calibration | Moderate |
| 110 | 3 / 4 / 15 | Engine Coolant Temperature – High or Voltage Abnormal | Moderate–High |
| 111 | 17 | Engine Coolant Level – Low | Severe |
| 172 / 174 | 3 / 0 | Fuel Temperature – Voltage Above Normal / Data Above Range | Moderate |
| 190 | 2 / 14 / 15 | Engine Speed Sensor – Erratic, Special Instruction, or High RPM | Moderate |
| 168 | 4 / 18 | Battery Power Input – Low Voltage or Below Normal Data Range | Moderate |
| 411 | 2 / 5 / 7 / 13 | EGR Differential Pressure – Data Erratic / Open Circuit / Mechanical Not Responding / Out of Calibration | Moderate–High |
| 412 | 2 / 3 | EGR Temperature – Data Erratic / Voltage Above Normal | Moderate |
| 652 | 7 | Ignition Timing – Mechanical System Not Responding or Out of Adjustment | Moderate |
| 637 | 8 | ECM Abnormal Frequency or Pulse Width | Moderate |
Insight: Most engine-related issues result from voltage fluctuations, corroded sensor terminals, or operator neglect of fluid maintenance intervals. These faults must be addressed promptly to prevent further internal wear.
2. Aftertreatment and Emission Control System Faults
| SPN | FMI | Description | Severity |
|---|---|---|---|
| C3556 | 1 | Aftertreatment Hydrocarbon Doser – Low Output | Severe |
| 4364 | 1 / 17 | SCR Catalyst Conversion Efficiency – Low, Least Severe | Severe |
| 5443 | 0/ 15 / 16 | Hydrocarbon Dosing System – High Output Variations | Moderate |
| 3719 | 0 / 15 / 16 / 31 | DPF Soot Load % – High or Unknown | Moderate–High |
| 3609 | 4 / 12 | DPF Voltage Below Normal or Sensor Device Fault | High |
| 3662 / 3362 | 5 / 31 | Aftertreatment Fuel Temperature – Open Circuit / Not Responding | Moderate |
| 3563 | 4 / 16 / 21 | Particulate Matter Sensor – Voltage Low, Parameter Missing, or Drifted High | Moderate |
| 3242 | 3 / 18 | Intake NOx – Voltage High / Below Normal Range | Moderate |
| 3360 | 9 | Exhaust Temperature – Abnormal Update Rate | Moderate–High |
| 3242 / 3226 | 10 | Outlet NOx Sensor – Abnormal Rate of Change | Moderate |
Summary: Clogged filters, contaminated DEF fluid, or sensor calcification often trigger these errors. Regular heater and dosing module inspections reduce FMI 16 and FMI 21 alerts.
For a deeper explanation of how emission-related codes combine with MID, CID, and FMI, see our technical resource on decoding Caterpillar diagnostic formats.
3. Transmission and Drivetrain Faults
| SPN | FMI | Description | Severity |
|---|---|---|---|
| 723 | 2 / 8 / 18 | Transmission Output / Range Position – Erratic, Abnormal, or Below Normal Range | Moderate |
| 729 | 3 / 5 / 18 | Transmission Range Position – Voltage High / Open Circuit / Below Range | Moderate |
| 751 | 9 | Primary Shift Selector – Abnormal Update Rate | Moderate |
| 639 | 2 / 9 | J1939 Network – Intermittent or Abnormal Update Rate | Moderate–High |
4. Fuel, Air, and DEF Systems
| SPN | FMI | Description | Typical Impact |
|---|---|---|---|
| 94 | 15 | Fuel Delivery Pressure – High, Moderate, Severe Level | Causes injector instability |
| 3509 | 4 | 5V Sensor Supply Bank 1 – Voltage Below Normal | Sensor voltage drop |
| 4334 | 18 | DEF Pressure Sensor – Below Normal Range | DEF low-pressure warning |
| 84 | 2 | Wheel-Based Vehicle Speed – Data Erratic | Affects traction control |
5. Protection, Safety, and Monitoring Systems
| SPN | FMI | Description | Severity |
|---|---|---|---|
| 4077 | 4 / 14 | Engine Protection System – Low Voltage or Special Instruction | High |
| 3660 | 5 | Engine Protection System – Current Below Normal | Moderate |
| 1110 | 31 | DOC Temperature – Not Responding | High |
| 1481 | 9 | Brake Control Device Update Rate – Abnormal | Moderate |
| 237 | 11 | VIN Detection – Other Failure Mode | Low |
| 168 (#repeated) | NULL / 4 | Warning Light On | Low |
Note: Most warning-light and power-input codes (SPN 168 family) stem from ECM grounding faults or battery resistance. These should be checked first before replacing expensive modules.
How to Read SPN FMI Codes?
Each SPN FMI code follows a consistent logic pattern:
- Identify the SPN (Parameter) to locate the subsystem.
- Identify the FMI (Mode) to pinpoint how the signal failed.
- Evaluate if the failure is electrical or mechanical.
- Cross-check for stacked codes—multiple faults from one source.
Example 1:
- SPN 639 FMI 9 – Communication network interrupted.
- Meaning: J1939 network data packets are not updating normally. This may freeze displays or cause transmission lag.
Example 2:
- SPN 4364 FMI 1 – SCR Catalyst Conversion Efficiency Low.
- Meaning: System detected insufficient NOx reduction; likely due to clogged dosing lines or malfunctioning sensors.
If warning screens show 4364-1 (red) or 3361-5 (yellow), as in the examples below, it signals a severe emission efficiency loss or “current below normal” event in dosing units.
Common Causes and Troubleshooting Tips
Most repeating SPN FMI code list alerts come from preventable electrical or calibration issues.
- Electrical Power & Ground Connections – Inspect ECM terminals, battery power supply, and cab connector seals.
- Sensor Calibration – Faults like FMI 13 (Out of Calibration) arise from aged sensors; recalibration tools can correct these.
- Contamination – Water or debris in fueling and exhaust dosing systems causes FMI 5 and FMI 16 alerts.
- Communication Delay – Codes 639 FMI 9 or 751 FMI 9 suggest CAN bus interference; check shield ground continuity.
- Software Updates – Outdated ECM logic may log false positives. Routine updates prevent misleading diagnoses.
When parts replacement is necessary, choosing high-quality aftermarket components keeps repair budgets manageable without sacrificing reliability. You can source sensors, filters, and hydraulic valves directly from CAT parts—offering OEM-quality components at affordable prices for loaders, dozers, and excavators.
How to Avoid Repeated Fault Codes?
To keep warning lights off and production on track:
- Perform Monthly ECM Health Checks – Clear obsolete codes and confirm real-time sensor feedback.
- Use Matching Parts Resistance Ratings – Replacement sensors or solenoids must meet OEM voltage tolerances.
- Maintain Clean Grounding Points – A rusted frame ground can trigger SPN 168 FMI 4 or 9 repeatedly.
- Schedule Regular Filter and DEF Line Cleaning – Prevent FMI 18 (low pressure) and FMI 21 (drifted high).
- Keep Network Integrity – Route wiring harnesses away from heavy-vibration points to avoid 639-series issues.
Summary
Decoding an SPN FMI code list isn’t just about clearing warnings—it’s about preventing mechanical downtime and avoiding costly replacements. Skilled operators interpret these codes early, combining diagnostics with high-quality aftermarket parts to maximize uptime. For robust and reliable components that fit your off-road heavy machinery, explore CAT parts from FridayParts—your one-stop source for OEM-quality, cost-saving solutions.
