Power supply require replaced.Short somewhere in power board.
Same problem on mine. I unscrewed the back and took a compressor hose and blasted 3 years of dust that had built up after being sucked in by the top fan. Then had to disconnect various panels from the obvious power distribution board. Then the TV started working fine after a few attempts. (Just unplugging the main power overnight fixed nothing and doing the above procedure seems to be only fix). I still have issue where it must have a signal from satellite box before the power on button responds. Also I have lost the on-screen guide since the analogue channels have gone. Planing on calling LG to get some fixes to the "power save" and guide issues. At least the power-off issue fixed for now!
So 2 weeks later after normal TV operation, the same power off after 3 seconds issue occured. This time removed the HDD only and the first power on - it shut down - but second attempt the TV has come on again. Has operated OK for 1 day now. I will update this post if the issue re-occurs. Right now the HDD is unplugged to see if that fixes the issue or is a symptom.
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It has something to do with a short circuit in either of the following boards:
It is a complicated task to find which is the culprit and a qualified technician or an experienced electronic technician may be needed to do it. The board with the short circuit can be found by means of isolation technique in which one board is disconnected one at a time and the TV is turned on. If the TV turns on and stays on, which can be determined by the presence of audio, light and/or partial video, that board is most likely the culprit. But further tests are required to confirm that it is the board with the short circuit.
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