The dreaded “SCR NOx conversion efficiency low” message on your Detroit Diesel DD13, DD15, or DD16 can signal costly repairs. This fault code indicates that the aftertreatment system, often referred to as the 1-Box, isn’t operating efficiently enough. Specifically, the SCR NOx conversion efficiency, measured between the inlet and outlet NOx sensors, has fallen below the required threshold of 70%. This article will delve into the intricacies of the Scr Nox Sensor, its role in emissions control, and the implications of low conversion efficiency.
What is the SCR NOx Sensor and How Does It Work?
The Selective Catalytic Reduction (SCR) system is crucial for reducing harmful nitrogen oxide (NOx) emissions from diesel engines. A key component of this system is the SCR NOx sensor. There are typically two NOx sensors: an inlet sensor positioned before the SCR catalyst and an outlet sensor located after. These sensors measure the NOx levels in the exhaust stream. The Engine Control Module (ECM) uses this data to calculate the conversion efficiency, representing the percentage of NOx converted into harmless nitrogen and water vapor.
Decoding the Fault Codes
When the SCR NOx conversion efficiency drops below 70%, the ECM triggers specific fault codes, including:
- EEC61 SPN 4364 FMI 1: Selective Catalyst Reduction NOx Conversion Very Low
- EEC61 SPN 4364 FMI 18: Selective Catalyst Reduction NOx Conversion Low
- EEC61 SPN 3364 FMI 2: Improper DEF Quality
These codes point to a problem within the aftertreatment system and require immediate attention. “EEC61” signifies an issue originating from the Aftertreatment Control Module (ACM), responsible for managing the Diesel Oxidation Catalyst (DOC), Diesel Particulate Filter (DPF), Diesel Exhaust Fluid (DEF), and SCR systems. A forced regeneration with a restored SCR NOx efficiency above 70% is necessary to clear these codes.
Causes of Low SCR NOx Conversion Efficiency
Several factors can contribute to low SCR NOx conversion efficiency:
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Contaminated DEF: Using contaminated DEF can hinder the chemical reactions necessary for NOx reduction.
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Incorrect DEF Concentration: The urea concentration in the DEF must be within a specific range for optimal performance.
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Faulty NOx Sensor: A malfunctioning NOx sensor can provide inaccurate readings, leading to incorrect efficiency calculations. This can include issues with the sensor itself or its wiring.
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Excessive DEF Buildup: DEF crystallization or buildup in the exhaust system can disrupt the SCR process.
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Failed SCR Catalyst: The SCR catalyst itself can degrade over time, reducing its ability to convert NOx. This is often referred to as a “failed 1-Box.”
Addressing Low SCR NOx Efficiency
Troubleshooting low SCR NOx efficiency involves several steps:
- Verify DEF Quality: Ensure you’re using high-quality DEF from a sealed container.
- Inspect NOx Sensors: Check both inlet and outlet NOx sensors for damage, contamination, or wiring issues.
- Assess DPF Function: Ensure the DPF system is operating correctly, as DPF problems can impact SCR performance.
Consequences of Ignoring the Problem
Ignoring these fault codes can lead to:
- Derate: The engine may enter a derate mode, limiting power output to protect the aftertreatment system.
- Breakdown: Continued operation with low SCR NOx efficiency can result in a complete system failure, requiring costly repairs.
Verifying the Fix
After addressing the potential causes, perform a forced regeneration. Successful completion of the regeneration and a restored SCR NOx efficiency above 70% confirms the issue is resolved. The fault codes should become inactive. Regular maintenance and prompt attention to fault codes are essential for ensuring the optimal performance and longevity of your Detroit Diesel engine’s SCR system. Remember, addressing these issues proactively can prevent costly downtime and keep your truck on the road.
