LG 42LV5500-UA LED TV turns on, but has no picture or sound

The led driver AND the T-con board are bad. If you could see a picture with the flashlight then the T-con board would be good. It is not. The led driver can be on the main board or the power supply board. You can get these parts from here: TV Parts Universal TV Stands DLP Chips ShopJimmy

It is a common problem for the LG model. The problem
is defective LED back lights ASSY. You have to open the display panel & replace the burned LED strips.

Fault at its main digital board. Short circuited component/s at it. Contact any service technician; or the authorized service center. If you wish to get some details; check the site linked here. Pull up older posts.
http://electronicshelponline.blogspot.com/
In most cases, the main board might have to be replaced. If so, a software [firmware] update too might be necessary.

Lamp switch is faulty replace the y board

Fault at its main digital board. Short circuited component/s at it. Contact any service technician; or the authorized service center. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts.
http://electro-medical.blogspot.com/
http://electronicshelponline.blogspot.com/
In most cases, the main board might have to be replaced. If so, a software [firmware] update too might be necessary.

the picture board has gone faulty
thee are two system boards in the set --one for sound and one for picture
if you have the number from the board you want to replace try e-bay --circuit boards for your tv and see what comes up

CHECK PSU VOLTAGES

My LG 26LX1D is the UA Version.

The solution for my problem was unclogging the Small Fan by the Main Panel, on the opposite side of the TV as the Power Board.

This is My Story.

The TV I found at the dump seemed good, but would not turn on.

Two LED's showed up on the front panel of the Television. First, the Red LED (on, off), then the Green LED (ON), then the Red LED (blinked) while the Green was still on.

I removed the back panel and saw that there were NO SIGNS OF BAD CAPACITORS on the Power Board. I plugged in the TV once more to see what would happen.

I noticed that there was a SMALL FAN which did not move when I plugged the power source to the outlet. This Fan was CLOGGED WITH DIRT.

I unscrewed the Fan from it's place and set it on a dish to act as a buffer against the main board. I then proceeded to clean it with a Vacuum Cleaner and a small Flathead Screwdriver.

I now own a FUNCTIONING LG 26LX1D-UA, RESSURRECTED!

I had same problem - no picture, but sound worked - LG 42PC3d - was two months past warranty, but LG agreed to fix it as a warranty call. Service guy was out within a week to replace the Z SUS(?) board. Works fine now. This must be a typical problem, as LG did fix it, even though the warranty had expired. Good luck.