A print system and a method for detecting inline failure of a curing light source of a 3D object printer are disclosed. For example, the print system includes a plurality of printheads, a curing light source, an inline failure detection (IFD) system, a movable member to hold an object, a heat resistant member coupled to the movable member that holds a thermal paper for testing operation of the curing light source and a controller to control movement of the movable member to move the object past the array of printheads, to operate the plurality of printheads to eject the marking material onto the object as the object passes the two-dimensional array of printheads, to operate the curing light source to apply energy to the thermal paper and to cure the marking material, and to operate the IFD system to read the thermal paper to detect a failure of the curing light source.