Many reviews on Amazon are not entirely accurate and are written by people who bought the wrong product. It's not the products. I can't comment on the seller's return policy, but I don't want to sell an item that people routinely buy by mistake. I'm no expert, but here's what I see in reviews and why they're wrong. The description clearly states that it is intended for use with the ECI-100, a Raymarine engine interface module. Without the ECI-100, this unit will not transmit your engine data to the GPS plotter.
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