Understanding and Resetting Selective Catalytic Reduction (SCR) System Faults in Your Vehicle
If you drive a modern diesel vehicle, chances are it is equipped with a Selective Catalytic Reduction (SCR) system to reduce harmful nitrogen oxide (NOx) emissions. While SCR technology is highly effective at curbing pollution, it can occasionally experience faults that trigger a warning light on your dashboard. In this comprehensive guide, we’ll dive deep into what the SCR system does, common issues that can arise, and step-by-step instructions on how to diagnose and reset SCR faults to keep your vehicle running cleanly and efficiently.
What is a Selective Catalytic Reduction (SCR) System?
Selective Catalytic Reduction is an advanced emissions control technology used in diesel engines to convert nitrogen oxides (NOx) into harmless nitrogen and water. SCR systems work by injecting a diesel exhaust fluid (DEF), usually a urea solution, into the exhaust stream where it reacts with a catalyst to break down NOx. By reducing NOx emissions by up to 90%, SCR allows vehicles to meet strict environmental regulations while maintaining performance and fuel efficiency. SCR is widely used in on-road and off-road diesel vehicles, including heavy-duty trucks, buses, construction equipment, agricultural machinery, and some passenger cars. It has become an essential technology for vehicle manufacturers to balance emissions requirements with engine performance demands.
Key Components of an SCR System
An SCR system consists of several main components that work together to reduce NOx emissions:
- DEF Tank: Stores the diesel exhaust fluid (urea solution) needed for the SCR process.
- DEF Injector: Sprays a controlled amount of DEF into the exhaust stream based on engine operating conditions.
- SCR Catalyst: A porous ceramic honeycomb coated with precious metals where the chemical reaction between DEF and NOx occurs, converting it into nitrogen and water.
- NOx Sensors: Monitors NOx levels before and after the SCR catalyst to determine how much DEF to inject and verify the system is functioning properly.
- Dosing Control Unit: An electronic control module that receives data from various sensors and controls DEF injection for optimal NOx reduction.
When all these components are working harmoniously, the SCR system can effectively reduce a vehicle’s NOx emissions to comply with environmental regulations. However, issues with any part of the SCR system can trigger a fault code and warning light, indicating the need for diagnosis and repair.
Common SCR System Faults and Causes
Several factors can lead to an SCR system malfunction, causing the check engine light to illuminate and potentially limiting engine performance. Some of the most common SCR faults include:
- Low or Contaminated DEF: The diesel exhaust fluid is crucial for the SCR system to function correctly. If the DEF level is too low or the fluid becomes contaminated, it can trigger an SCR fault. Contamination can occur due to using the wrong type of fluid, exposing DEF to extreme temperatures, or allowing foreign substances to enter the tank.
- Clogged or Malfunctioning DEF Injector: Over time, the DEF injector nozzle can become clogged with crystallized urea, preventing it from spraying the proper amount of fluid into the exhaust stream. A faulty injector wiring harness or control valve can also cause inconsistent DEF dosing, leading to an SCR fault.
- Damaged or Failing NOx Sensors: NOx sensors are responsible for monitoring the effectiveness of the SCR system. If a sensor becomes damaged or fails due to age or exposure to extreme heat, it can send inaccurate readings to the dosing control unit, triggering an SCR fault.
- Catalyst Efficiency Loss: The SCR catalyst can lose efficiency over time due to thermal aging, poisoning from sulfur or other contaminants, or physical damage. As the catalyst degrades, it becomes less effective at reducing NOx emissions, eventually causing an SCR fault.
- Dosing Control Unit Issues: Problems with the dosing control unit, such as faulty wiring, software glitches, or water intrusion, can disrupt communication between sensors and the DEF injector. This can lead to incorrect DEF dosing and an SCR fault. When an SCR fault occurs, the engine control module (ECM) stores a diagnostic trouble code (DTC) and illuminates the check engine light to alert the driver. The specific DTC can help pinpoint the root cause of the problem during diagnosis.
Diagnosing SCR System Faults
If your vehicle’s check engine light is on due to an SCR system fault, the first step is to retrieve the diagnostic trouble code (DTC) using an OBD-II scanner. Some common SCR-related DTCs include:
- P20EE: SCR NOx Catalyst Efficiency Below Threshold
- P20E8: SCR Inducement System Malfunction
- P249D: Closed Loop Reductant Injection Control At Limit – Flow Too Low
- P20E9: Reductant Level Too Low
- P20F4: Reductant System Performance
Once you have the DTC, consult the vehicle’s repair manual or an online database for a description of the code and potential causes. This will give you a starting point for further diagnosis.
Visual Inspection
Begin by visually inspecting the SCR system components for any obvious damage, leaks, or loose connections. Check the DEF tank and lines for cracks, the DEF injector for signs of crystallization, and the wiring harnesses for frayed or corroded wires.
DEF Quality Test
If the DTC points to a problem with the diesel exhaust fluid, test the DEF using a refractometer to ensure it meets the required concentration of 32.5% urea. If the concentration is too low or the fluid appears contaminated, drain the tank and refill it with fresh, high-quality DEF.
NOx Sensor and Wiring Tests
For DTCs related to NOx sensors, use a multimeter to test the sensor’s resistance and continuity per the manufacturer’s specifications. Also, check the sensor wiring harness for any damage or loose connections that could cause intermittent readings.
Injector and Dosing Control Unit Diagnostics
If the fault points to the DEF injector or dosing control unit, connect an advanced diagnostic tool to monitor live data streams while running the engine. This can help identify any discrepancies between the commanded and actual DEF injection rates or communication issues between the control unit and other components.
Catalyst Efficiency Evaluation
Diagnosing a failing SCR catalyst typically requires measuring NOx levels before and after the catalyst using a gas analyzer. If the post-catalyst NOx levels are consistently higher than expected, it indicates the catalyst is no longer effectively reducing emissions and may need replacement. By systematically testing and inspecting each component of the SCR system, you can pinpoint the root cause of the fault and take appropriate action to restore proper function.
Resetting SCR System Faults
Once you have diagnosed and repaired the issue causing the SCR fault, you’ll need to clear the diagnostic trouble code (DTC) and reset the system to turn off the check engine light. The specific steps may vary depending on your vehicle make and model, but here’s a general guide:
- Ensure Successful Repair: Before attempting to reset the SCR fault, double-check that the underlying issue has been fully resolved. Clearing the DTC without fixing the problem will likely cause the fault to reappear shortly after.
- Locate the OBD-II Port: The On-Board Diagnostics (OBD-II) port is typically located under the dashboard on the driver’s side of the vehicle. It is a 16-pin connector that allows communication between the vehicle’s computer and diagnostic tools.
- Connect an OBD-II Scanner: Plug a compatible OBD-II scanner into the vehicle’s OBD-II port. Make sure the scanner is designed to work with your specific vehicle make and model.
- Turn the Ignition On: Turn the vehicle’s ignition to the “On” position without starting the engine. This will power up the OBD-II scanner and allow it to communicate with the vehicle’s computer.
- Clear the Fault Code: Using the OBD-II scanner, navigate to the “Erase Codes” or “Clear Fault Codes” function. Select the SCR-related DTC you wish to clear and confirm the action. The scanner will send a command to the vehicle’s computer to erase the stored fault code.
- Verify Fault Clearance: After clearing the fault code, use the OBD-II scanner to recheck for any active DTCs. If the SCR fault has been successfully cleared, no related codes should appear.
- Test Drive and Monitor: Take your vehicle for a test drive, keeping an eye on the dashboard for any warning lights. Monitor the SCR system’s performance using the OBD-II scanner or the vehicle’s information display to ensure the fault does not reoccur. In some cases, additional steps may be required to reset the SCR system, such as performing a forced DPF regeneration or resetting the DEF quality sensor. Consult your vehicle’s repair manual or a professional technician for specific guidance on these advanced procedures.
Preventing SCR System Issues
While SCR faults can be frustrating, there are several steps you can take to minimize the risk of system malfunctions and keep your vehicle’s emissions control running smoothly:
- Use High-Quality Diesel Exhaust Fluid: Always use a high-quality DEF that meets ISO 22241 specifications to ensure optimal SCR system performance. Avoid using agricultural-grade urea or other fluids not specifically designed for automotive SCR systems, as they can cause damage and trigger faults.
- Maintain Proper DEF Levels: Regularly check your vehicle’s DEF tank and top it off as needed to prevent the system from running low. Most vehicles will display a warning message when the DEF level drops below a certain threshold, giving you time to refill the tank before a fault occurs.
- Keep the DEF Tank and Components Clean: When refilling the DEF tank, take care to avoid introducing contaminants such as dirt, dust, or other fluids. Clean the area around the tank’s filler neck before opening it, and use a dedicated DEF funnel to prevent accidental contamination.
- Address Warning Lights Promptly: If your vehicle’s SCR system warning light illuminates, don’t ignore it. Investigate the cause of the fault and take appropriate action as soon as possible to prevent further damage or emissions issues.
- Follow Recommended Maintenance Schedules: Adhere to your vehicle manufacturer’s recommended maintenance schedule for the SCR system, including regular DEF filter replacements and sensor inspections. Staying on top of preventive maintenance can help catch potential issues early and extend the life of your SCR components.
- Plan for Cold Weather: DEF can freeze at temperatures below 12°F (-11°C), which can cause issues with the SCR system. Most vehicles are equipped with DEF tank heaters to prevent freezing, but it’s still a good idea to keep the tank at least half full during cold weather to minimize the risk of faults. By following these best practices and addressing SCR faults promptly, you can ensure your vehicle’s emissions control system remains effective and compliant with environmental regulations.
The Importance of Proper SCR System Maintenance
Maintaining a functioning SCR system is crucial not only for the environment but also for your vehicle’s performance and longevity. Here are some key reasons why proper SCR maintenance is so important:
- Compliance with Emissions Regulations: Vehicles equipped with SCR systems are designed to meet strict emissions standards, such as the US Environmental Protection Agency’s (EPA) Heavy-Duty Diesel Emissions Standards or the European Union’s Euro 6 regulations. Failing to maintain your SCR system can lead to non-compliance with these regulations, potentially resulting in fines or legal consequences.
- Avoiding Vehicle Derate: Many modern diesel vehicles are programmed to enter a “derate” mode if the SCR system malfunctions or runs out of DEF. In derate mode, the engine’s performance is intentionally reduced to limit NOx emissions, which can significantly impact your vehicle’s power, speed, and fuel efficiency until the issue is resolved.
- Preventing Catalyst Damage: Neglecting SCR system maintenance can lead to more serious problems down the line, such as irreversible damage to the SCR catalyst. Contaminated DEF, clogged injectors, or faulty sensors can cause the catalyst to become poisoned or overheated, reducing its effectiveness and shortening its lifespan. Replacing an SCR catalyst can be a costly repair, so it’s best to avoid this situation through proper maintenance.
- Maintaining Fuel Efficiency: A well-functioning SCR system not only reduces NOx emissions but can also help improve your vehicle’s fuel efficiency. By optimizing the combustion process and allowing the engine to run at its most efficient settings, SCR technology can contribute to lower fuel consumption and cost savings over time.
- Protecting the Environment: At its core, the purpose of an SCR system is to minimize the environmental impact of diesel emissions. By reducing harmful NOx pollutants, SCR helps combat smog, acid rain, and respiratory health issues. Keeping your vehicle’s SCR system in top condition is an important way to do your part in protecting the environment and public health.
Conclusion
Selective Catalytic Reduction systems have become an essential technology for reducing harmful emissions from diesel vehicles while maintaining performance and efficiency. However, like any complex system, SCR can experience faults that require prompt attention to avoid more serious issues. By understanding how your vehicle’s SCR system works, recognizing common fault symptoms, and following proper diagnostic and maintenance procedures, you can keep your emissions control running smoothly and comply with environmental regulations. Regular DEF top-offs, sensor inspections, and adherence to manufacturer guidelines are key to preventing SCR faults and extending the life of your system components.
If you do experience an SCR fault, don’t hesitate to investigate the cause and take appropriate action to resolve the issue. Whether it’s replacing a faulty sensor, cleaning a clogged injector, or replenishing contaminated DEF, addressing SCR problems early can save you time, money, and headaches in the long run. As emissions standards continue to tighten around the world, the importance of properly maintaining SCR systems will only grow. By staying informed and proactive about your vehicle’s emissions control, you can do your part to reduce air pollution, protect the environment, and keep your diesel engine running at its best.