The top left quadrant of my LED tv display is dark. It looks like the LEDs are out. What is the most likely culprit when only 1/4 of the display is dark? Board or LEDs themselves?

This model had issues with cold solder connections going from the power to LED driver board, I think you will see the issues once you remove the power supply, the 2 boards plug directly into each other, its the power supply plug that gives issues

I'd give you a 90% chance of bad LED backlights. There have been some issues with LED drivers but most the time it's the backlights, Vizio does things a bit different than other brands, most sets if a strip goes bad the entire set is dark but vizio drives each string separately so it's possible to have sections dark on the screen

With only part of the display being dark, the problem is the individual led lights. They usually come in strips that you can purchase from places like shopjimmy,com

If all of the back lights are light up, the problem mostly is the display panel. we have seen a few of them.

The most common problem with backlights in an LED TV set is a failed LED. While these are supposed to last 30,000 - 50,000 hours, they are very sensitive to static electricity damage. This does not show up immediately, but after some time, the LED may flicker, then burn out, or burn out suddenly.

On many LED TV models, the backlight will come on, then turn off if one of the LED strips is bad. Test procedure: unplug the TV set, wait 10 seconds, then plug it back in (or turn off the power strip for 10 seconds if you have the TV plugged into a power strip). Darken the room, then turn on the TV while watching the screen carefully. If you see a horizontal section of the TV that is darker than the rest of the screen before the screen goes completely dark, that is where the bad LED strip is.

If none of the LEDs light up, even briefly, the problem may be a bad LED driver circuit. This is often part of the power supply board, but may be a separate board. The LED driver board is identifiable by being connected to a cable of three or more single wires (not twisted pairs or flat ribbon) going into the metal back pan of the TV, generally to the upper left side (viewed from the back). If the driver is separate from the power supply board, it will have another cable going to the power supply board..

Repairing a bad LED is labor-intensive.
1. Disconnect the TV from everything and remove the back cover. (at this point, you can plug in the set and repeat the LED test, and watch the various holes in the metal back pan to see if the LEDs flash when you turn on the set. Be very careful - there are exposed high voltage connections on the power supply board when the TV is plugged in, even if the set is off.
2. Make sure the TV is unplugged. Remove the shield from the T-Con board (at the top or bottom of the TV, centered horizontally).
3. Unlatch the flat cable connectors between the T-Con board and the edge of the LCD display.
4. Treat a section of clean carpet or large beach towel with anti-static spray, then lay it on a flat surface (make sure there are no foreign objects in this area; you don't want anything punching into the LCD). Lay the TV face down on this mat.
5. Remove the stand if it is still attached.
6. Remove all of the screws from the perimeter of the LCD. At this point, it is very desirable to have a helper. Lift the TV upright, holding it by both the front bezel and the metal back pan. Remove the front bezel (on most models it is held on by plastic tabs with hooks as well as some of the screws that were just removed).
7. Check for another set of screws that hold a rectangular steel frame or set of steel strips to the edge of the screen. Lay the TV flat again and remove these.
8. Very carefully unfold the connector strip(s) with the T-Con cables from the frame and lay them flat on the mat. Avoid any stress on the ribbons between the strips and LCD glass; if you tear or pull loose any of them, the display is ruined. Avoid touching any of the metal circuit tracing on the strips or the bare metal on the ends of the T-Con cables - static electricity discharge may damage the display.
9. Gently lift the back part of the TV (the metal pan with the circuit boards attached) off of the display. Two people are recommended for this task, especially for larger sets. On some models, it may be more workable to remove the LCD and diffuser sheets one part at a time while the TV is held upright. Make sure to keep all of the layers in the proper order, including front-side alignment, and put them on a clean lint-free surface.
10. There may be a white plastic reflector pan covering the LED strips in the back pan. The plastic is typically very thin and delicate. Carefully remove it.
11. Now you should have the LED strips exposed. At this point, you can plug in the TV and turn it on while watching the LED strips. (Caution: there are exposed high voltages on the back of the TV. Do not touch the power supply board while the TV is plugged in!) You will see the good LED strips flash while one or two strips remain dark. The bad LED is on one of the dark strips.

At this point, you have several options: 1. Replace the entire set of LED strips. If one LED has been abused (excess current or static electricity), chances are others in the set are also damaged. 2. Replace just the failed strip(s); 3. Identify and replace the bad LED; or. 4. Replace all of the LEDs in the bad strip(s). The strips are usually held in place by double-sided tape or a tacky adhesive. LED replacement requires tools for soldering surface-mount devices. If there is a circular piece of white plastic tape around the LED, carefully peel it off and save it, and likewise pull off the lens that is glued on over the LED before attempting to remove the LED.

White LEDs are blue LEDs with yellow phosphor painted on top. Blue LEDs are more vulnerable to static damage than other colors due to their strained-crystal construction. Consequently, parts with blue or white LEDs should not be handled unless electrostatic discharge safety measures are practiced.

I would need to guess so since this is your money I would spend I will not say.

Check the connections of the flat cables that connects the T'Con board to LCD pane for any loose contact. Re-seat them. Visit the site named 'Electro-help' for more details. Click this link. View it in "Timeslide" or "Flipcard" option, which can be selected from the drop down box menu by the [Sidebar] tag, at the top left corner of the display window. You can also click the [Home] tab to see the latest updates. It will make search easy. There are more than 1350 posts to different brand TVs now. Pull up older posts there or make a search by type in the brand name of the device in the "Search Box", at the top right of the display window.

You need to give a more detailed description of the problem.
The LG 47LH90 is an LED backlit TV with 'local dimming' technology. This means that there are lots of white LEDs behind the screen and these can be controlled in groups to allow localised areas of the screen to be dimmed to produce better contrast between light and dark. It is very likely that when the room is dark, this local dimming is more noticeable. The actual individual LEDs cannot be controlled and so the area that is dimmed will generally be greater than the area that is required to be dark 'blooming effect'. In addition, LED backlit displays only maintain their contrast through a tight viewing angle. If you sit too farto the right or left, then light from non-dimmed LEDs will leak through and the screen will look lighter.
In short, there may not be any fault with your TV.

i unhook everything and cleaned the system and it worked fine

I think try going to display setting under control panel. See if it allows you to change to full screen mode.