What happens to the transponder if Mode C fails?

When Mode C of a transponder fails, it typically reverts to Mode A. Mode A transponders provide a basic function by transmitting a four-digit identification code, but they do not provide altitude data, which is the key feature of Mode C. This automatic reversion is part of the transponder's design, ensuring that if more advanced features fail, it can still provide essential identification information to air traffic control.

The transition to Mode A allows the aircraft to maintain some level of communication with radar systems and continue to be recognized, although without the altitude information that Mode C provides. This is crucial for the safety and coordination of air traffic, especially in busy airspace.

In contrast, shutting down completely would eliminate all the transponder functions, leading to a loss of radar contact. Continual normal operation without altitude reporting would not occur since the failure of Mode C means that the altitude information is lost, and switching to an alternate frequency is not a function typically associated with transponders regarding Mode C failures.