An Evaporative Emission Control System (EVAP) testing device using a control unit connected to a gas cap vehicle interface unit. The gas cap vehicle interface unit is configured to match the required threading for the gas tank receptacle on a vehicle and has a pressure bypass port. The pressure bypass tube or channel extends through the gas cap vehicle interface unit to a pressure sensor module in the control unit. The control unit includes electronics needed to capture and transmit the pressure typically using a wireless transmission component. The control unit also includes a power switch and power indicating light. The control unit and gas cap vehicle interface unit can be combined into one unit or separate units designed to be connected to each other.