Text Box: TYPICAL SYMPTOMS 1. REPLY LAMP OPERATES IN ALL MODES AS DISCUSSED IN MAIN TROUBLESHOOTING PAGE. 2. REPLY LAMP COMES ON IN TEST MODE. 3. REPLY LAMP FLASHES IN FLIGHT. 4. ATC REPORTS YOUR TRANSPONDER HAS FAILED IN BOTH MODE A, AND MODE C. OR (LESS COMMON) 5. ATC REPORTS FAILURE OF ONE MODE (USUALLY MODE C, ALTITUDE).  THIS WOULD INDICATE THE TRANSMITTER IS WORKING AND THE MODE C GATE IS DEFECTIVE.