Compact text indication of lost radio connection

I am working on a UI for a robotics application that uses radio communication. There is a text field for each robot that normally displays battery voltage in the format 3.72 V.

Sometimes the radio communication link breaks, so we can not query the battery voltage. I want to indicate this to the user place of the battery voltage. I am trying to choose a message of around 5 characters for this purpose.

If the radio connection is lost, most operations with the robot will fail. Our initial design used N/A (not applicable / not available) in the field to indicate this, but in my opinion N/A connotes more of a "warning" condition than an "error" condition.

Aviation uses INOP to indicate subsystems of an aircraft that are inoperative, but this is probably less well-known.

The ideal message would:

• Be a "call to action", telling the user they probably need to fix something
• Indicate that the radio has failed as opposed to a generic "something's wrong"
• Be well-known

Of course I can make the UI more spacious or use icons, but if there is a good short textual message it will be easier to implement.