I had an HDS 8 unit where the gps didnt register anymore. It was out of warranty, but called up lowrance, and they sent me a “service pack” replacement….basically got a new HDS8 gen 2 and all the hardware for under 400 bucks. My bad unit had a serial number though. I would think you should be albe to locate the serial number in the system settings somewhere…I got the service pack replacement on my 2nd attempt calling Lowrance though. I got no where with the first attempt….I called back after I got my replacement just to see if the person I drew on the phone knew anything about the service pack replacement, and they didnt. I guess the support staff is not all on the same level, and it depends who you get. I would call them back and see if the tech can guide you on how to retrieve the serial number and go from there.