Brand new Fishmark 480, transducer installed according to directions, demo mode works great. When I got out on the lake yesterday and turned it on, it would read water temp ( temp display was flashing), but the depth scale went to 100 ft. and the graph showed nothing! Unit would also shut down at random. Same results with motor on, or off. I did notice the battery seemed low, the big motor was sluggish turning over.. could this be the problem?? Any ideas or suggestions are appreciated.

Had the same issue with my 320 DF, battery power was the problem, charged the battery on the boat for a while and it came back on.

Thanks, I'll charge it up and give it a try this weekend

You might be able to top off a bad battery having a bad cell that will allow temporary opeartion of the sonar. If it's weak it won't hold enough charge to run the sonar but will allow sluggish cranking which doesn't require a full 11-12 volts like the sonar wants, mostly needing a bolt of current with as low as 9 volts. I'd have a double load test put on the battery. If you are powering the sonar off the cranking battery and cranking is suffering, you've found your problem.

Jim