The Dosing Control Unit (DCU) is a vital component of modern emission control systems, particularly in diesel engines employing Selective Catalytic Reduction technology (SCR). Its primary function is to accurately control the injection of urea or Diesel Exhaust Fluid (DEF) into the exhaust stream to reduce nitrogen oxide (NOₓ) emissions. This paper presents the architecture, operation, diagnostic features, and innovation of a newly developed DCU system. The Engine Control Unit, using real-time data from sensors monitoring parameters such as exhaust temperature, NOₓ levels, and engine load, calculates the required DEF dosage. Based on DEF dosing request, the DCU activates the AdBlue pump and air valve to deliver the precise quantity of diesel exhaust fluid needed under varying engine conditions. The proposed system adopts a master-slave configuration, with the ECU as the master and the DCU as the slave. The controller design emphasizes cost-effectiveness and simplified hardware, and software architecture compared to commercial counterparts. Additionally, it integrates diagnostic functions compliant with On-Board Diagnostics (OBD) and Unified Diagnostic Services (UDS) standards to detect system anomalies such as blockages, leaks, and electrical faults (e.g., short to ground or battery). By combining accurate dosing with advanced diagnostics, the DCU enhances SCR system efficiency, fuel economy, and compliance with strict emission norms. Test bench and simulation results confirm that the developed controller meets international emission and diagnostic standards. This positions the DCU as a significant contributor to cleaner, more efficient, and sustainable automotive technologies.