ARLA 32 and SCR System: How It Works and Common Issues

Introduction

When Brazil adopted phase P7 of PROCONVE, equivalent to Euro 5, in 2012, diesel trucks began to feature an environmental ally — and a new source of service calls: the SCR system with ARLA 32.

How the System Works

ARLA 32 is a 32.5% urea solution in water. It is neither toxic nor explosive and is classified as a low‑risk fluid for transport. It resides in a dedicated tank that typically needs refilling every 3 to 5 diesel tanks.

1. A pump delivers the solution to the dosing valve.
2. The ECU calculates the dosage based on engine data.
3. ARLA is injected into the exhaust pipe.
4. In the SCR catalyst, NOx is converted into regular nitrogen and water vapor.

The system only activates when the exhaust reaches the required temperature.

Components Worth Monitoring

• Urea pump and dosing unit
• Tank temperature and level sensor
• Temperature sensors before and after the catalyst
• NOx sensor with its control unit
• Compressed‑air system and purge routine (cleaning)

The Role of the NOx Sensor

The NOx sensor monitors nitrogen‑oxide concentration in the exhaust gases to ensure emissions stay within limits. It operates similarly to a wide‑band lambda probe: it uses a ceramic body, oxygen‑pumping cells, and an integrated heater to reach operating temperature quickly. The control unit calculates the raw NOx value and sends a digital signal over the CAN bus to the SCR module and the engine ECU.

Practical Application

• Confirm ARLA level and quality before any test.
• Check fault codes from the SCR modules and the engine.
• Verify power, heating, and communication of the NOx sensor before replacing it.
• Inspect the dosing unit and its lines.

Tip: An SCR fault is not an injection fault, but it often limits engine power. The customer perceives it as a “engine problem.”

Conclusion

Those who understand SCR serve a fleet that grows each year and position themselves as complete modern‑diesel specialists.

In today’s diesel world, knowing electronics and after‑treatment is part of the diagnosis.