8 Signs Your SCR System Needs Attention: Warning Indicators to Watch For

Parked vehicles in a lot

Selective Catalytic Reduction (SCR) systems are crucial components in diesel engines, designed to reduce harmful emissions like nitrogen oxides (NOx) by converting them into harmless nitrogen and water vapor.

Over time, SCR systems may require maintenance due to various factors. Continuous exposure to high temperatures and corrosive exhaust gases can lead to the deterioration of SCR components, necessitating inspections and repairs.

Regular maintenance, including DEF tank refills, filter replacements, and periodic inspections, ensures optimal SCR functionality and compliance with emissions regulations.

This guide outlines signs that indicate your SCR system requires maintenance.

8 Signs That Indicate It’s Time for SCR System Maintenance

Several signs may indicate that an SCR system requires attention:

1. Decreased Performance

A noticeable reduction in engine power or acceleration may suggest an issue with the SCR system. Since the SCR system relies on the proper conversion of NOx emissions, any malfunction can affect engine performance. This symptom may manifest as sluggishness or difficulty reaching desired speeds.

2. Warning Lights

Modern vehicles are equipped with onboard diagnostics systems that monitor various components, including the SCR system. If the system detects a fault or abnormality in the SCR system, it will trigger warning lights on the dashboard. These lights may include the check engine light or a specific SCR system warning light, indicating the need for immediate attention.

3. Reduced DEF Levels

Diesel exhaust fluid (DEF) is a crucial component of the SCR system, as it helps facilitate the conversion of NOx emissions. A decrease in DEF levels without a corresponding increase in mileage suggests a leak or malfunction in the SCR system. Monitoring DEF levels regularly can help detect potential issues early on.

4. Increased Exhaust Emissions

One of the primary functions of the SCR system is to reduce harmful emissions, particularly nitrogen oxides (NOx). If the SCR system is not functioning correctly, there may be an increase in NOx emissions or visible smoke coming from the exhaust. This can indicate that the SCR catalyst is not effectively converting emissions as intended.

5. Engine Derating

Engine derating occurs when the vehicle’s onboard computer detects a fault in the SCR system that could potentially harm the engine or emissions control system. To protect the engine, the computer may limit power output, leading to reduced performance. This is a safety measure to prevent further damage until the issue is addressed.

6. Strange Smells

Ammonia-like smells emanating from the exhaust can indicate a malfunctioning SCR system. DEF, which contains urea, is converted into ammonia during the SCR process. If the SCR system is not operating correctly, excess ammonia may be released into the exhaust, resulting in an unusual odor.

7. Poor Fuel Economy

The SCR system’s malfunction can impact overall engine efficiency, leading to decreased fuel economy. If the engine is not operating optimally due to SCR system issues, it may require more fuel to achieve the same level of performance, resulting in poorer fuel efficiency.

8. Exhaust System Noise

Unusual noises coming from the exhaust system, such as rattling, hissing, or metallic sounds, may indicate a problem with SCR components like the catalyst or injector. These noises can result from physical damage, clogging, or leaks within the SCR system and should be investigated promptly to prevent further damage.

White car on a city street
A sleek white car parked on a city street, symbolizing the need for regular vehicle maintenance to ensure safety and performance.

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