During the 3D printing process errors can occur and go unnoticed until it reaches the user due to the lack of closed loop control. The low-end FFF printers usually suffer more of these failures caused by absence of material or movement of the filament interrupted by different reasons: The filament spool gets tangled causing that the filament cannot reach the extrusion nozzle, the motor that feeds the extruder stop working, or an external object is blocking the movement of the filament. To address these, the work reported takes the first steps towards closing the loop: supervising the extrusion nozzle, detecting material flowing out, and correlating it to the printing process to determine if it is occurring when it should. The supervision is performed with a camera pointing at the nozzle and an interface has been developed to select one of 10 common filament colors that can be monitored. The system then process the image captured and identifies only the filament as it's coming out of the nozzle, enabling to detect discontinuities on the material flow. Although the system is being developed for FFF printers, the need for feedback and closing the loop on the printing process would benefit all additive manufacture processes; therefore, the work proposed is seen as a test case that can later be scaled to other printing technologies.