Lg 55 in powering on has a black screen then powers of by itself

mjzot first you need to rule out your inputs as error causing. Remove all of them (especially if you have some sort of cable box, sat box etc.)Unplug your TV and unplug all input sources. Hold down the power button for 30-60 sec. Then reconnect one input source (preferably local like Game console, DVD/BR player). and make sure you use a known-well HDMI cable, if that's how you connect. Lastly reconnect the TV and try to turn it on. See if that changes anything.

First off my suspicion is that this is caused by failed backlight strips. LG LN series has a high backlight failure rate.

Try this. Disconnect the T-con board, unplug the TV and plug it back in and turn on to see if backlights are visible through the small holes in the back panel. The backlight should come on by unplugging the power cord and then plugging it back in with the TV on. The T-con board is the one under the small metal cover that has the flat ribbon cable from the main board connected to it. Center bottom on your picture.

One more thing to try, with everything else back on, would be to disconnect the backlight.brown board, top connector, small cable going into the panel (top center) and then start your TV. SEE if the start up behavior has changed.

Next remove the back from your TV ;-) Get a multimeter and get the Voltage readings on the power supply connectors that go to the main board. Measure those with the TV OFF and then measure again with the TV ON. OF importance is the voltage on the PWR-ON (PS-ON) and DRV-ON (BL-ON)? You should get 12V/24V when PWR-ON signal from main board is present. After that you will get back lights when PWR-ON, DRV-ON, and PDIMs are present. Okaty, now let us know what you find out.