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It sounds as if your Sonar Transponder is not giving you depth indications.
Not a problem with a 'chip' at all. It might be something wrong with the transmission of the sonar transponder's signals.
Check the cable to that sonar transponder along its full length, for cuts, severe bending, or other damage.
With the power to the system Off:
Undo each connection along the route from GPSMap 4210 to sonar transponder, one at a time, scrub each connector with a clean dry brush for 20+ seconds, then wipe each side of the connector with a clean dry rag cloth until the metal shines, and re-connect firmly, then on to the next connector.
Then power it On, and see if the Sonar now reads the Depth OK.