A practical reference for understanding and responding to common emission system fault codes in construction equipment.
Table of Contents
- Understanding Fault Codes
- Fault Code Structure
- DEF System Codes
- SCR System Codes
- DPF Related Codes
- Sensor Fault Codes
- Code Severity Levels
- What To Do When Codes Appear
- Conclusion
Understanding Fault Codes
Modern construction equipment uses standardized fault codes to communicate problems with the emission system. Understanding these codes can help you determine the severity of an issue and whether it requires immediate attention or can wait for scheduled service.
Why Codes Matter
- Early warning: Codes often appear before symptoms become severe
- Diagnostic direction: Points technicians to the right system
- Compliance tracking: Documents emission system status
- Prevent damage: Early intervention prevents costly failures
Fault Code Structure
J1939 SPN/FMI Format
Heavy equipment typically uses SAE J1939 standard codes with two components:
- SPN (Suspect Parameter Number): Identifies the component or system
- FMI (Failure Mode Identifier): Describes the type of failure
Example: SPN 3361 FMI 4 = DEF Pressure (3361) is Low (4)
Common FMI Meanings
| FMI | Meaning |
|---|---|
| 0 | Data Valid But Above Normal Range |
| 1 | Data Valid But Below Normal Range |
| 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 |
| 9 | Abnormal Update Rate |
| 11 | Root Cause Not Known |
| 12 | Bad Intelligent Device or Component |
| 13 | Out of Calibration |
| 14 | Special Instructions |
| 31 | Condition Exists |
DEF System Codes
DEF Tank and Supply
| Code | Description | Common Causes | Action |
|---|---|---|---|
| SPN 1761 | DEF Tank Level Low | Low fluid, faulty sensor | Check/refill DEF, inspect sensor |
| SPN 3361 FMI 4 | DEF Pressure Low | Pump failure, leak, clogged filter | Check pump, lines, filter |
| SPN 3361 FMI 0 | DEF Pressure High | Blocked injector, restricted line | Check injector, clear restrictions |
| SPN 3364 | DEF Tank Temperature | Heater failure, sensor fault | Check heating system, sensor |
| SPN 5392 | DEF Dosing Unit | Injector fault, wiring issue | Inspect injector, check wiring |
DEF Quality Codes
| Code | Description | Common Causes | Action |
|---|---|---|---|
| SPN 1761 FMI 0 | Poor DEF Quality Detected | Contaminated or diluted DEF | Drain and refill with quality DEF |
| SPN 5246 | DEF Quality Sensor | Sensor failure, contamination | Clean or replace sensor |
SCR System Codes
SCR Catalyst and Efficiency
| Code | Description | Common Causes | Action |
|---|---|---|---|
| SPN 3226 | SCR NOx Conversion Efficiency | Catalyst degradation, sensor fault, DEF issue | Check DEF quality, inspect catalyst |
| SPN 4364 | SCR Operator Inducement | System requires service attention | Address underlying SCR faults |
| SPN 4765 | SCR Thermal Management | Temperature out of range | Check exhaust temps, sensors |
| P20EE | SCR Catalyst Below Threshold | Aged catalyst, sensor issue | Diagnose catalyst condition |
DPF Related Codes
DPF Status and Regeneration
| Code | Description | Common Causes | Action |
|---|---|---|---|
| SPN 3251 | DPF Soot Load High | Insufficient regen, excess soot | Perform forced regeneration |
| SPN 3720 | DPF Differential Pressure | Soot/ash accumulation, sensor fault | Check sensor, consider DPF service |
| SPN 3701 FMI 16 | DPF Moderately Blocked | Soot buildup, failed regen | Forced regen required |
| SPN 3701 FMI 0 | DPF Severely Blocked | Excessive soot, regen impossible | Professional cleaning or replacement |
| SPN 3699 | DPF Regen Inhibited | Conditions not met for regen | Check inhibit conditions (PTO, idle, etc.) |
Caterpillar Specific DPF Codes
| Code | Description | Action |
|---|---|---|
| 3701-16 | Moderate DPF Blockage | Initiate parked regen with Cat ET |
| 3701-0 | Severe DPF Blockage | Dealer service required |
| 3719-3 | DOC Inlet Temp High | Check for exhaust restriction |
Komatsu Specific Codes
| Code | Description | Action |
|---|---|---|
| E9200 | DPF Regeneration Fault | Check regen conditions, may need dealer |
| E9201 | DPF System Fault | Diagnose with Komatsu software |
Sensor Fault Codes
NOx Sensors
| Code | Description | Common Causes | Action |
|---|---|---|---|
| SPN 4094 | NOx Sensor 1 (Inlet) | Sensor failure, wiring | Inspect wiring, test sensor |
| SPN 4095 | NOx Sensor 2 (Outlet) | Sensor failure, wiring | Inspect wiring, test sensor |
| P2201 | NOx Circuit Range/Performance | Sensor reading abnormal | Check sensor operation, replace if needed |
| P229E | NOx Circuit Short to Ground | Wiring short, damaged sensor | Check wiring, replace sensor |
| P220A | NOx Sensor Heater | Heater element failure | Replace sensor (heater not serviceable) |
Temperature Sensors
| Code | Description | Common Causes |
|---|---|---|
| SPN 3242 | DPF Inlet Temperature | Sensor fault, wiring issue |
| SPN 3246 | DPF Outlet Temperature | Sensor fault, wiring issue |
| SPN 4360 | SCR Inlet Temperature | Sensor fault, wiring issue |
| SPN 4363 | SCR Outlet Temperature | Sensor fault, wiring issue |
Code Severity Levels
Level 1 - Monitor
Continue operating but schedule service soon:
- Low DEF level warnings
- Minor sensor drift codes
- Regen needed notifications
Level 2 - Service Soon
Address within days to prevent escalation:
- SCR efficiency warnings
- DPF soot load high
- Intermittent sensor faults
Level 3 - Service Required
Stop and perform service when safe:
- Forced regen required
- DEF quality fault
- Component failures
Level 4 - Stop Engine
Continuing operation may cause damage:
- DPF severely blocked
- Engine derate active
- Inducement countdown active
What To Do When Codes Appear
Immediate Steps
- Note the code: Record exact code number and any message
- Check basics: DEF level, fuel level, visible damage
- Assess severity: Warning light color, engine performance
- Document conditions: What was machine doing when code appeared
When You Can Continue Operating
- Amber warning light only
- No power reduction
- Low DEF level (refill soon)
- Regen request (allow regen when possible)
When To Stop and Service
- Red warning light
- Power derate active
- Multiple codes present
- Countdown timer displayed
- Unusual sounds, smoke, or smells
Information to Provide Technicians
- All fault codes displayed
- When problem started
- Operating conditions at time of fault
- Recent service or fuel/DEF fill
- Hour meter reading
Conclusion
Understanding SCR/DEF fault codes helps you respond appropriately to emission system problems in your construction equipment. While some codes indicate minor issues that can wait for scheduled service, others require immediate attention to prevent damage or extended downtime.
Always record codes accurately and provide complete information to service technicians. This helps ensure efficient diagnosis and repair, getting your equipment back to work faster.
Need help diagnosing emission system fault codes? Our technical team specializes in construction equipment SCR/DEF systems.