The transducer cable has a bad power connection socket

Question edited for clarity.

Make sure you got the correct transducer Your model comes in single frequency and dual frequency. Make sure the connection on the back of the unit is good and you have no corroded or missing pins. Make sure the software is up to date.

Lowrance LMS-525C DF Fish Finder Parts

Product Description: Designed for salt water or downrigger tracking, this dual-frequency sonar/GPS chartplotter
provides additional 50 kHz readings, in addition to a high-res 480 x 480 display on a 5" screen
and many professional features.

* Dual-frequency 50/200 kHz
* Precision 12-parallel channel external GPS+WAAS receiver
* Lowrance radar compatible
* LowranceNET® networking (NMEA 2000® certification pending)
* High-speed ethernet connection links multiple Lowrance display units and modules to
operate independently on shared network data, including sonar
200 khz transducer PDRT-WBL Part #106-89
Shoot-thru-hull mount, 20 degree pod type with remote temperature (200 kHz). (20 ft cable) Dual Frequency Transducer HP-DFBL260 Part #106-82
Thru-hull-mount, dual frequency bronze housing, high-power multiple element (50 kHz 19 degree
& 200 kHz 6 degree).

HST-DFSBL Part #106-77
Transom-mount, dual frequency skimmer with built-in temperature sensor. (25 feet of cable)

https://www.google.com/search?q=lowrance+lms-525c+df+software+update

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From your crank battery...

run a resistance test on the connecting cable as the wire can become corroded inside the insulation and break contact

You lost your power wires + & - or blown a fuse. The other two are the transducer connections.

Lowrance site shows the power cable as a plug-in. Did you unit not have one? Take it back!

You either have a faulty head unit, bad connection between the transducer and head unit, or a faulty transducer or transduced cable.

verify cable harness integrity at base connection of transducer , replace cable or check settings easy fix would be change display unit if unit under warranty as replacing transducer is not easy and economical

now there is a chance that the transducer is bad ,, but I would check the battery voltage and make sure you have a soild 12 volts to the unit. also in my many years of working with and installing these units on boats I have see the needle pins ( where the plug inserts in the back of the display unit ) bend and/or break off completely. look in the plug area of the display unit and make sure all pins are intact. one more place to look for the problem is in the plug itself, take the plug apart and inspect the wire connection and make sure they are all good. if after all the above has been checked then I would assume that it is the transducer itself.
if you have anymore questions with this problem, please feel free to write back and ask,

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Double check you have a good connection from the transducer data cable to the back of the display unit and that the pins are not corroded or anything similar. If the temp has never worked at all, it may be because the transducer you are using doesnt have a temperature sensor onboard and you will need to get one that does have this feature. If it did work previously and you checked the pins/connection (as per above) then the transducer is faulty or has developed a fault. There is no real easy fix and it will most likely be far cheaper just to buy a new transducer than to have the current one repaired by Eagle. I think new trasnducers are about $60-$70.

Disconnect cable from back of unit clean the pin connections on the unit and on the cable. If rusty clean with Corrosion X are WD
IF it doesnt work replace cable Jim OMC Retired Mech