Urea solution plays a crucial role in reducing nitrogen oxide (NOx) emissions within diesel engine exhausts through selective catalytic reduction (SCR) systems. Control over the injection of urea solution is essential to ensure a complete reaction with the NOx concentration before the SCR catalyst and that the NOx concentration does not exceed the allowable threshold. In this study, a closed-loop proportional integral derivative (PID) control method was used using Proteus software. Input data entered into Proteus software includes NOx concentration after and before the SCR catalyst, exhaust temperature, and exhaust flowrate taken from sensors. Output parameters, including urea injection flowrate and NOx conversion efficiency, are calculated and displayed on the LED screen. The study has determined the urea injection map when changing NOx concentration before the SCR catalyst, exhaust flowrate, and exhaust temperature.