Complete troubleshooting guide for the most common Freightliner diagnostic trouble codes. Get proven repair solutions that reduce downtime and keep your fleet running efficiently.
Proven repair procedures for the most frequent Freightliner DTCs, saving hours of diagnostic time.
These codes require immediate attention to prevent engine damage or safety issues. Understanding critical vs non-critical codes is essential for proper triage.
| Code | Description | Common Fix | Severity | Downtime |
|---|---|---|---|---|
| SPN 3362 FMI 31 | Aftertreatment 1 DEF Quality | Drain DEF tank, refill with fresh DEF, clear codes. Check for SCR issues | Critical | 2-4 hrs |
| SPN 625 FMI 9 | CAN Communication Error | Check J1939 backbone, inspect terminating resistors. See CAN bus guide | Critical | 1-3 hrs |
| SPN 4364 FMI 18 | SCR NOx Conversion Low | Replace NOx sensors, update ECM calibration, check DEF doser | Critical | 3-5 hrs |
| SPN 3251 FMI 0 | DPF Soot Load High | Force regeneration, check for intake/boost leaks, verify DPF differential pressure sensor | High | 1-2 hrs |
| SPN 110 FMI 16 | Engine Coolant Temperature High | Check coolant level, inspect thermostat, test cooling fan operation | Critical | 2-4 hrs |
| SPN 1569 FMI 31 | Engine Protection Torque Derate | Address underlying fault causing derate. Check derate conditions | Critical | Varies |
| SPN 102 FMI 18 | Intake Manifold Pressure Low | Check turbocharger operation, inspect charge air cooler for leaks | High | 2-3 hrs |
| SPN 157 FMI 18 | Fuel Rail Pressure Low | Replace fuel filter, check fuel pump, inspect for air in fuel system | Critical | 1-3 hrs |
| SPN 3226 FMI 13 | NOx Sensor Out of Calibration | Replace NOx sensor, perform SCR efficiency test. Reference sensor guide | High | 1-2 hrs |
| SPN 639 FMI 13 | J1939 Network #1 Communication Error | Inspect data link connectors, check ECM connections, verify network resistance | Critical | 2-4 hrs |
Detroit Diesel engine codes frequently encountered in Freightliner vehicles
Fix: Replace fuel filters, check lift pump operation
Time: 30-60 minFix: Check fuel cooler, verify fuel return flow
Time: 1-2 hrsFix: Test relay, check wiring harness
Time: 45-90 minFix: Clean EGR valve, check actuator operation
Time: 2-3 hrsFix: Test injector resistance, check connector
Time: 1-2 hrsFix: Replace MAP sensor, check for shorts
Time: 45-60 minFix: Check turbo wastegate, inspect CAC
Time: 1-3 hrsFix: Inspect VGT actuator, check for exhaust restrictions
Time: 2-4 hrsFix: Check charge air cooler efficiency
Time: 1-2 hrsFix: Clean EGR cooler, check differential pressure
Time: 3-4 hrsCommon DT12 automated transmission fault codes
| Code | System | Description | Quick Fix | Est. Time |
|---|---|---|---|---|
| SPN 523 FMI 7 | Clutch | Clutch Actuator Not Responding | Check clutch actuator air supply, inspect solenoids | 1-2 hrs |
| SPN 524 FMI 2 | Gear Select | Gear Selector Position Erratic | Calibrate shift actuator, check position sensor | 45-90 min |
| SPN 560 FMI 14 | Software | Transmission Calibration Required | Perform adaptation learn procedure using diagnostic tools | 30-45 min |
| SPN 584 FMI 9 | Communication | TCU Communication Lost | Check TCU power and ground, inspect CAN connections | 1-3 hrs |
| SPN 177 FMI 3 | Oil System | Transmission Oil Temperature High | Check oil level, inspect cooler operation | 1-2 hrs |
| SPN 3359 FMI 2 | Input Shaft | Input Speed Sensor Intermittent | Clean speed sensor, check air gap | 30-60 min |
| SPN 3360 FMI 2 | Output Shaft | Output Speed Sensor Erratic | Replace speed sensor, verify tone ring condition | 45-75 min |
| SPN 591 FMI 19 | Network | Transmission CAN Bus Error | Check J1939 connection at TCU | 1-2 hrs |
| SPN 2003 FMI 31 | Clutch Life | Clutch Wear Limit Reached | Schedule clutch replacement | 6-8 hrs |
| SPN 5018 FMI 14 | PTO | PTO Engagement Fault | Check PTO solenoid, verify interlock switches | 1-2 hrs |
Common electrical system and body controller faults
Battery Voltage High
Fix: Check alternator regulatorBattery Voltage Low
Fix: Test alternator output, check beltsAlternator Not Charging
Fix: Replace alternator or regulatorLeft Headlamp Circuit
Fix: Check bulb, test relay and fusesRight Headlamp Circuit
Fix: Inspect connector, check groundTurn Signal Circuit Open
Fix: Replace flasher relayAC Compressor Circuit
Fix: Check clutch coil resistanceBlower Motor Fault
Fix: Test motor, check resistor packCabin Temp Sensor Error
Fix: Replace temperature sensorCritical safety system diagnostic codes requiring immediate attention
Complete ABS system malfunction detected
Fix: Diagnose ABS ECU, check all wheel speed sensors
Safety Critical - Do not operateESC steering input calibration required
Fix: Perform steering angle sensor calibration
Stability control disabledErratic signal from RF wheel sensor
Fix: Clean sensor, check tone ring for damage
ABS function impairedOpen circuit in LR wheel speed sensor
Fix: Check connector, test sensor resistance
ABS disabled on affected wheelInsufficient brake system pressure
Fix: Check air system, inspect for leaks
Stop vehicle immediatelyFront radar blocked or misaligned
Fix: Clean radar sensor, check alignment
Safety features limitedESC yaw sensor communication fault
Fix: Check sensor mounting, verify CAN connection
Stability control unavailableBrake pedal position signal erratic
Fix: Adjust or replace brake switch
Cruise control disabledMultiple ABS faults detected
Fix: Perform complete ABS system diagnosis
Schedule immediate serviceCommunication lost with trailer ABS
Fix: Check trailer cable, verify power at connector
Trailer ABS inactiveUse this guide for rapid triage decisions. For detailed procedures, see our on-road triage guide
Engine damage imminent or safety critical failure
Continue to destination then repair
Address at next PM interval
Log and track for patterns
Expert answers to help you troubleshoot faster
Codes return when the underlying problem hasn't been fixed. The ECM continuously monitors systems and will set codes again if fault conditions persist. Some codes require specific drive cycles to clear completely. Always address the root cause before clearing codes. Use data logging to capture intermittent faults. For aftertreatment codes, ensure you've completed any required regeneration cycles.
Active codes indicate current faults that are happening now and will illuminate the MIL (check engine light). Inactive codes are historical - the fault occurred but isn't currently present. Always fix active codes first. Review inactive codes for patterns that might indicate intermittent problems. Understanding code structure helps differentiate between code types. Some inactive codes clear automatically after 40 fault-free ignition cycles.
Yes, but with limitations. Derates are designed to protect the engine while allowing you to reach a service location. Power and speed will be reduced based on severity - typically 25%, 40%, or 55% torque reduction. Severe derates may limit speed to 5 mph. Review our derate rules guide for specific conditions. Address the underlying issue promptly to prevent progression to more severe derates or engine shutdown.
Detroit Diesel Diagnostic Link (DDDL) is the OEM software for Detroit engines. For complete vehicle diagnostics, use DiagnosticLink for Freightliner-specific systems. Generic tools like Nexiq or Noregon JPRO work but may not access all proprietary codes. For remote diagnostics, Detroit Connect provides real-time monitoring. Other brands use different tools - Volvo uses PTT, while Kenworth uses DAVIE4.
Scan for codes during every PM service, pre-trip inspection if equipped with onboard diagnostics, and immediately when the MIL illuminates. Set up live monitoring for continuous scanning. Document all codes, even inactive ones, to track patterns. Many fleets scan weekly or bi-weekly as preventive maintenance. Critical codes should trigger immediate alerts to prevent breakdowns.
Essential Freightliner DTC resources for comprehensive troubleshooting
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