That's what we been trying to do. I think he may be using a HD SI transducer with a non side imaging unit. It could just be a settings issue, such as transducer selection. You can not just snap the unit where another one was any more. As far as the GPS, it's the same for all manufacturers and Humminbird developed a heading sensor to help those who need this feature, without charging the majority who do not. I personally don't want to pay the extra 200 and if I did feel the need, I would not pay the extra on a second unit. I would just network the 2 together. Two units on same boat would make me think it may be a installation problem, such as low voltage or maybe trolling motor interference. Just want to help it is some thing simple. We just have to locate the problem.
I would not go in or let anyone into that unit it will void a 3 year warranty as Eufala is the only warranty center. Please try it on someone's boat who has the exact same unit working there. Or explain exactly your problem with pictures here. Make sure the unit is wired directly to the battery, and originally packaged transducer is being used. A trolling motor will give interference. We really want to help.