Navigating fault codes on your Detroit Diesel DD13, DD15, or DD16 engine can be daunting, especially when you encounter warnings related to your Selective Catalyst Reduction (SCR) system. Among the most concerning is the “SCR NOx conversion efficiency low” fault, often accompanied by the code 4364. This error signals a problem within your aftertreatment system, commonly known as the 1-Box, and it’s crucial to address it promptly to avoid costly repairs and downtime.
In essence, the “SCR NOx conversion efficiency low” fault code, specifically SPN 4364 FMI 1 or FMI 18, indicates that the 1-Box is not operating at the required efficiency level, which should be above 70%. This efficiency is determined by comparing readings from the inlet and outlet NOx sensors. When the SCR NOx efficiency dips below this 70% threshold, these fault codes become active and illuminate your dashboard. Clearing these codes necessitates a forced regeneration cycle performed when the SCR NOx efficiency is back above 70%.
Let’s delve into the technical details to better understand this issue and how to tackle it.
Decoding the SCR NOx Conversion Efficiency Low Fault
When the dreaded check engine light illuminates with an SCR NOx efficiency fault, you’ll likely see codes similar to these, particularly in EPA10 & GHG14 models (2011-2016):
- 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 (Often related and worth checking)
EEC 61 points to the Aftertreatment Control Module (ACM), the brain managing your DOC, DPF, DEF, and SCR system operations. These codes are your engine’s way of telling you the ACM has detected a problem with NOx conversion efficiency.
Understanding the Description
The core issue is that the ACM has determined the SCR NOx conversion efficiency is low. This calculation is performed by measuring NOx levels before (inlet) and after (outlet) the SCR catalyst. If the efficiency rate, derived from these sensor readings, falls below a pre-set threshold, the fault code is triggered. This threshold is critical for meeting emission standards and ensuring optimal engine performance.
Possible Causes of Low SCR NOx Conversion Efficiency
Several factors can contribute to a low SCR NOx conversion efficiency reading. Identifying the root cause is the first step towards resolving the issue. Here are some common culprits:
- Contaminated DEF: Diesel Exhaust Fluid (DEF) purity is paramount. Contaminants can interfere with the SCR process, reducing its effectiveness.
- Incorrect DEF Concentration: The urea concentration in DEF needs to be precise (typically 32.5%). Incorrect concentration throws off the chemical reactions within the SCR catalyst.
- Biased NOx Sensors: Faulty or inaccurate inlet or outlet NOx sensors can provide incorrect readings to the ACM, leading to a false low efficiency fault or masking a real issue.
- Excessive DEF Build-up in the Exhaust: While DEF is essential, excessive build-up due to issues in the DEF dosing system can hinder the SCR process and sensor readings.
- Failed 1-Box (Aftertreatment System): The most serious cause is a failure within the 1-Box itself, which encompasses the DOC, DPF, and SCR components. Catalyst degradation or damage necessitates replacement.
Boosting SCR NOx Efficiency Above 70%
If your SCR NOx efficiency is below 70%, getting it back up is crucial for clearing the fault codes and ensuring your Detroit Diesel runs smoothly. Here’s a troubleshooting approach:
- Verify and Replace DEF: Start with the basics. If there’s any doubt about your DEF quality, replace it immediately. Use DEF from a sealed container to minimize contamination risk.
- Check NOx Sensors: NOx sensors are vital for efficiency calculation. Diagnose the inlet and outlet sensors to ensure they are functioning correctly. Biased or failing sensors must be replaced.
- Assess DPF System Operation: A healthy Diesel Particulate Filter (DPF) system is interconnected with SCR efficiency. DPF issues, such as a clogged filter or problems with regeneration, can indirectly impact NOx conversion. Address any underlying DPF problems.
Is This a Common Problem?
Unfortunately, yes. “SCR NOx conversion efficiency low” is a frequently encountered issue, especially in Detroit Diesel EPA10 & GHG14 trucks manufactured between 2011 and 2016. The complexity of aftertreatment systems and the sensitivity of SCR catalysts contribute to this commonality.
Consequences of Ignoring the Fault Code
Ignoring “SCR NOx conversion efficiency low” fault codes is highly inadvisable. These are not minor warnings. Continued operation with these active codes will likely lead to:
- Engine Derate: The engine control system may reduce engine power to protect the emission system and comply with regulations.
- Breakdowns: Unaddressed SCR issues can escalate, leading to system failures and unexpected downtime.
- Increased Repair Costs: Delaying repairs often results in more extensive and expensive fixes down the line, potentially requiring full 1-Box replacement.
Verifying the Repair
Once you’ve addressed the potential causes, how do you confirm the fix? The key is to achieve an SCR NOx efficiency above 70%. Afterward, initiate a forced regeneration. If the regeneration cycle completes successfully and the fault codes become inactive, you’ve likely resolved the issue.
NOx Sensor Locations on Detroit Diesel 1-Box
To aid in diagnosis and sensor replacement, understanding the location of NOx sensors is helpful. Here are visual guides:
Keep Your Detroit Diesel Running with Confidence
Dealing with “Scr Nox Conversion Efficiency Low 4364” doesn’t have to leave you stranded. Understanding the fault, its causes, and troubleshooting steps empowers you to take informed action. For further assistance in managing your fleet and minimizing downtime, consider tools like the OTR Diagnostics tool. It’s designed to help you clear fault codes, reset your DPF and SCR systems, and avoid unnecessary trips to the dealership.
We hope this guide provides valuable insights into resolving this common Detroit Diesel issue. Stay safe and keep your trucks rolling!
