Chosen Solution
There are three possible reasons that would cause you to face this issue; Power board is faulty and/or dead. This would require a replacement of the boardMain board is faulty and/or dead. This would also require a replacement of the board.Backlight LED Strips are fault and/or dead. This would require a complete disassembly of the T.V. to expose the LED Strips in order to manually test each LED to ensure they are working. The issue is that it could be either one of these three issues or it can be a series of them. I would recommend that you start with the power board then the main board ending with the backlight as the process involved to repair the backlight LED strips requires advanced technical knowledge and time.
this is usually caused by a open led in the panel
I had a problem on a similar model. The mainboard needed a solder reflow, specifically, the XD CPU chip under the big heatsink. I wrote up the process in detail at https://lab.remaker.com/2020/11/lg-47lw5…, but it’s the same as any BGA reflow: Remove the heatsink to expose the troublesome chip, optionally add flux, shield the surrounding area with foil, heat the bottom, and use an oven reflow of your choice. I used a cheap heatgun, with the 550 F setting (low), heating the underside of the board for a little over a minute, and then heating the top side for four minutes, with the heatgun always moving in small circles. You can also take the board to a repair shop to get the chip professionally removed and re-balled. eBay has a number of services that will do this by mail.
