There are a number of things the pilot can do before he determines that the fault is within the transponder unit itself. The aircraft transponder is just one unit in the whole transponder system. The system also includes the aircraft battery, alternator, wiring, circuit-breakers, ground connections, coaxial cable, connectors, antenna, antenna ground plane, and even the antenna's position, or placement, on the airframe.
Pay close attention to what the transponder unit is doing and what the symptoms are. When you first turn it on, older units will briefly light the "reply light" during warm-up while newer units may have a digital display. If you don't see this, suspect a power problem. Check the circuit breaker, the battery voltage, the connectors at the back of the mounting tray, and the ground wire. Sometimes simply removing the unit from the panel and then plugging it back in can "clean" and reseat connector pins.
Perhaps the encoder is causing the problem. Just unplug it from the back of the transponder and see if that helps. If ATC controllers can now see your Squawk code, then replace the encoder.
If its getting proper voltage, but the reply is weak or non-existent, check the antenna. The transponder antenna is usually on the belly of the aircraft and looks like a 3-inch nail with a little ball on the end or maybe a fiberglass "blade", also about 3-inches long. These antenna can get very dirty with oil and dirt rendering them ineffective. Clean it carefully with soap and water and try again. Make sure its firmly attached to the airplane. Sometimes removing it and cleaning the mounting screws and the area where it attaches can solve a grounding problem.
Carefully check the coaxial cable all the way from the back of the unit to the antenna. Look for any kinks, sharp bends, or chaffing against airframe structure. If you have an ohmmeter, check for continuity at each end of the center conductor and each end of the outer shielding. If it needs to be replaced, be sure to replace it with the correct cable type. At these frequencies (1090 MHZ) some R.F. cables have too high a signal loss.
