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Even after thorough rinsing my GoPro Hero 5 has visible salt buildup on the gasket of the battery. Mine wont start properly after 1 week of not using it.
You may have internal corrosion on the Motherboard of the camera. Follow the guide below and take the motherboard up, and upload some images of the Motherboard so we can determine of there is corrosion or not. GoPro Hero5 Black Motherboard Replacement Adding images to an existing question
