What indicates that the Mode C reported altitude is corrupt or lost?

The indication that Mode C reported altitude is corrupt or lost is characterized by Field C being vacant. In air traffic control, Field C typically refers to the altitude reporting field that provides the Mode C altitude information from an aircraft. If this field is empty, it signifies that the altitude data is not being received, indicating a potential issue with the aircraft's altitude reporting system or the transmission of that information.

Conversely, a situation where the controller-entered reported altitude is valid does not necessarily reflect the status of the Mode C altitude. Additionally, the absence of Mode C data being received or a status of VFR (Visual Flight Rules) does not specifically point to the corrupt or lost nature of the Mode C reported altitude—it could simply indicate a different operational context. Therefore, a vacant Field C is a direct and clear representation of the issue with the Mode C altitude.