Chosen Solution
Remove the shield bracket and make sure the connector is seated properly. If you have a multimeter check the pin in red it should be 0.53 with red probe on ground, also check the corresponding pin on the flex cable, it should go to ground when the power button is pressed. If you haven’t got a multimeter and the connectors look fine I would replace the power button flex assembly.
Hello, does the power button still click and not register a click, or does it feel stiff? It could either be a bad connection to the motherboard (check the flex cable and connect on the LB - located on the top right under a shield secured by two screws) -maybe it was knocked when the battery was replaced? Otherwise, it could be that the flex cable itself is damaged and need replacing, which is more extensive than a basic battery replacement, but can be done using an iFixit guide and the part itself is under $10 from reputable sellers on eBay, so it’s not that expensive. I am curious to know if the volume buttons and mute switch work? Hope this helps (:
there are two connectors left one is for volume and silent key and right one for power key
