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The Garmin echoMAP 50s is stated to be compatible with any Garmin Panoptix™, any HD-ID™, any CHIRP-enabled, Garmin DownVü™ transducer or NMEA 2000®-compatible depth transducer.
The Airmar model P79 is stated to be an 'entry-level' transducer. In other words, it is kind of basic, a 600 Watt sounder with no frills aside from its adaptability to various mounting angles.
I suspect that you simply speaking have an incompatible transducer. (Or doubtfully, a 'broken' transducer.) The Garmin echoMAP 50s won't work with it for that reason.