An electric circuit assembly for transmitting a signal to initiate the inflation of a gas bag safety assembly in an automobile when the vehicle is involved in a collision. The circuit assembly is provided with diagnostic means to allow the driver to monitor the readiness of the circuit for operation and detect any malfunction in the assembly. Upon start-up of the vehicle, indicator means in the circuit assembly, in the form of a pair of warning lamps, indicates whether the circuit assembly is operating normally. One of the lamps is normally lit for a short period after the ignition switch is closed. The other lamp only becomes lit when the first lamp fails to light. The circuit is formed so as to operate properly despite the failure of various of its components.