Through hull transducer

[MT Pockets]

Any info on how to troubleshoot the through hull transducer for the Raymarine E-80, that monitors sea water temp and depth? The depth seems ok, but the sea water temp shows -38 F all the time. The unit itself seems to be a 4-5 inch long by 1 1/2 inch round plug that goes through a collar in the hull. attached to it is a "plug" to replace the unit when removed from the hull. Is this unit serviceable or do you just replace the whole unit? If so how do I determine the proper replacement item #? Thanks for any ideas.