What indication on the RMU suggests a possible transponder failure?

The indication of all dashes in the ATC code field on the RMU suggests a possible transponder failure because this display indicates that the transponder is not properly providing its code to the air traffic control system. In normal operation, the ATC code field should display a four-digit transponder code, which is essential for identifying the aircraft to air traffic controllers. When all dashes are visible, it typically means the transponder has either failed, is not receiving power, or is not configured correctly, leading to a loss of the aircraft's identification information.

In contrast, flashing lights on the RMU may indicate notifications or alerts, but they are not specifically tied to transponder performance. Incorrect altitude information could point to issues with the altimeter or other systems rather than the transponder. A GPS status error message indicates a problem with GPS functionality and does not directly relate to the transponder's operation. Thus, the clear indication of all dashes in the ATC code field is a direct sign of a potential transponder issue.