OK, this may help some, I turned my Sony KLD 550 back-light from 5 to 7 and the TV instantly turned off.
Then went into an endless startup loop because the back-light is failing and the TV will not run with it set to 7. The red light at the front of the unit confirmed there is an issue with the back-light because it flashes 6 times at each boot-up. It would boot to the Sony Logo & would then switch off & start all over again endlessly. This is because the auto light sensor is telling the TV to brighten the back-light back to 7 because it is in a dark room but the back-light cant handle it so it shuts down. I could not even do a hard reset using the TV buttons because it was so far out of wack & the TV could not boot far enough to let me do it. I tried for hours doing every hack on YouTube & the forums but nothing fixed it.
Time to think outside the Box.
Now U might think this is BS but to get around this issue and get the TV to the point where I could do a hard reset I got a super bright LED Torch & shined it straight into the light sensor on the front of the TV at boot-up. This made the TV turn down the Back-light at boot-up,,, which allowed the TV to finally well,, Boot-Up. Then I instantly got it to do a hard reset and when this was done I turned the back-light down to 4 & put brightness on MAX . It runs like a dream again now & I can hardly tell the difference in brightness or picture quality.
At least I don’t have to go without TV and it gives me time to buy a new one now.