If your Freightliner truck equipped with a DD13 or DD15 engine won’t accelerate past 55 mph, you’re likely experiencing a derate condition. This speed restriction, often accompanied by fault codes like Spn 521050 Fmi 31, is a protective measure to prevent damage to your aftertreatment system. This article will delve into the causes, symptoms, and solutions for this common issue.
Aftertreatment System: The Primary Culprit for Speed Restrictions
A 55 mph speed limitation usually indicates a problem within the aftertreatment system, responsible for reducing harmful emissions.
Symptoms:
- Flashing red DEF (Diesel Exhaust Fluid) light
- Empty or low DEF gauge reading
- Initial 25% power derate limiting speed to 55 mph, potentially progressing to a 5 mph derate
- Active diagnostic trouble codes (DTCs), such as SPN 521050 FMI 31
Causes of a 55 mph Derate:
Several components within the aftertreatment system can trigger a derate:
- Empty DEF Tank: Refill the DEF tank immediately.
- DEF Pump Failure: The DEF pump delivers DEF to the aftertreatment system. A faulty pump requires replacement.
- NOx Sensor Failure: NOx sensors monitor the efficiency of the emissions system. Malfunctioning sensors necessitate replacement.
- ACM (Aftertreatment Control Module) Power Loss: This disrupts communication between the engine control modules and the aftertreatment system.
- Damaged Aftertreatment Wiring Harness: Wiring issues can cause intermittent faults.
- Outdated Software: Ensure your engine and aftertreatment system software is up-to-date.
Affected Engines, Years, and Models
- Engines: Detroit Diesel DD13 and DD15
- Years: 2011-2021
- Emissions Standards: EPA10, GHG14, GHG17
- Models: Freightliner Cascadia, Western Star, and other Freightliner models using these engines
Understanding Fault Codes: SPN 521050 FMI 31 and Others
Specific fault codes pinpoint the issue. SPN 521050 FMI 31, often preceded by SPN 521049 FMI 31, indicates a DEF inducement CEL (Check Engine Light) and a significant derate, limiting speed to 5 mph. Other relevant codes include:
- SPN 1231 Fail 9 (ACM communication errors)
- SPN 4364 Fail 1/18 (Low NOx conversion efficiency)
- SPN 3364 Fail 2 (Improper DEF quality)
- SPN 521052 FMI 31 (Similar to SPN 521050 FMI 31)
DIY Troubleshooting Steps
- Check DEF Fluid Level: Ensure the DEF tank is full.
- Read and Clear Fault Codes: Use a diagnostic tool to identify and clear the codes.
- Perform a Forced DPF Regeneration: This process cleans the Diesel Particulate Filter and can resolve some aftertreatment issues. Specialized tools, like OTR Diagnostics, can initiate this procedure.
Resolving the Derate: Next Steps
While basic troubleshooting can sometimes resolve the issue, persistent problems, especially those involving fault codes like SPN 521050 FMI 31, require professional diagnosis and repair. Consult a qualified technician with experience in Freightliner aftertreatment systems to pinpoint the root cause and perform the necessary repairs. Addressing the underlying issue promptly will get you back on the road safely and efficiently.
