The dreaded “Scr System Fault Engine Will Derate” message can bring a Volvo truck to a screeching halt, limiting speed to a frustrating 5 mph. This derate, activated by the engine’s computer, is a protective measure to prevent damage to the engine or Diesel Particulate Filter (DPF) due to emissions system issues. Understanding the system and common causes can help you get back on the road quickly.
What Triggers the “SCR System Fault Engine Will Derate” Message?
The Selective Catalytic Reduction (SCR) system, implemented in Volvo trucks from 2011 onwards, plays a crucial role in reducing harmful exhaust emissions. A 5 mph derate specifically points to a problem within this system. Several components within the SCR system can trigger this fault:
Key Components of the SCR System and Potential Failures:
- DEF Fluid: Low DEF fluid levels (SPN 5392, SPN 1761) or poor DEF quality can trigger a derate. The DEF tank has sensors to monitor both level and quality. Bad DEF fluid can contaminate the system.
- DEF Doser Valve: This valve (SPN 5394) precisely controls the injection of DEF fluid into the SCR catalyst. A malfunctioning valve disrupts this process, leading to a derate.
- DEF Pump Assembly: Responsible for circulating DEF fluid throughout the system, a pump failure (SPN 4334, SPN 4339) will cause an immediate derate due to lack of pressure.
- NOx Sensors: Inlet and outlet NOx sensors (SPN 4094) measure nitrogen oxide levels in the exhaust. If these sensors fail or detect excessive NOx, it indicates an issue with the SCR system’s efficiency.
- ACM (Aftertreatment Control Module): This module manages the entire emissions system, communicating directly with the Engine Control Module (ECM). An ACM failure, often due to harness issues, can cause a derate, display a datalink error, and show an empty DEF gauge.
- EGR System: Malfunctions in the Exhaust Gas Recirculation (EGR) system can impact NOx efficiency and contribute to SCR problems.
Specific Fault Codes for SCR Inducement
The “SCR Inducement” fault directly triggers the 5 mph derate. Specific fault codes vary depending on the truck’s model year and emissions standard:
- 2011-2013 EPA10 Trucks: SPN 5246 FMI 0 – SCR Inducement Final Level
- 2014-2019 GHG14, GHG17 Trucks: P208E, P207F, P103B, P103C – DEF SCR Inducement Faults
Temporary Override (Use with Caution)
In some Volvo trucks, temporarily unplugging the transmission speed sensor might override the 5 mph derate. This sensor is typically located at the transmission tail shaft or on the right side of the rear transmission for I-Shift models. Disconnecting it will disable the speedometer and limit RPM to around 1400. Gear changes will need to be manual. This is not a recommended solution, only for emergency situations, and may not work on all models. Some trucks are programmed to maintain the 5 mph limit even with the sensor disconnected.
Addressing the Root Cause
Resolving the “SCR system fault engine will derate” requires diagnosing the underlying cause of the “SCR Inducement” code. This involves using diagnostic software to pinpoint the faulty component. In addition to the common failures listed above, consider:
- Software Updates: ECM/ACM software updates can address known issues and improve system performance. While not a guaranteed fix for all problems, staying updated is recommended.
Conclusion
A “SCR system fault engine will derate” message signifies a serious issue with your Volvo truck’s emissions system. Understanding the components involved and potential causes can help in diagnosis and repair. While a temporary override might be possible in emergencies, addressing the root cause is crucial for long-term reliability and performance. Consulting with a qualified Volvo technician and utilizing diagnostic tools are essential for proper diagnosis and repair.
