If an ATC transponder has not been tested in a specified period, what usage limitation applies?

When an ATC transponder has not been tested within the specified maintenance period, its operation is prohibited to ensure the safety and reliability of air traffic control communications. It is crucial that transponders function correctly to transmit accurate information about the aircraft's position, altitude, and identification to air traffic controllers. Using an untested transponder could lead to miscommunications and potentially dangerous situations in controlled airspace. Regulatory standards stipulate strict maintenance and testing intervals for these devices, and failure to comply leads to an outright ban on the transponder's usage until it has been properly tested and certified.