Sony Bravia KLV-40BX400 flicker no proper image but sound input good

@marlondale58257 that could be a panel or a main board (GPU) error. Your TV passes its POST and no error code blink from the power LED which is good and bad. Your TV does not know it is not working. Common errors would cause the power LED to give you trouble code.

We will need to see your boards and the associated wiring. Unplug your TV and open the back cover. Then take some good pictures and post those with your Question. That way we can see what you see. Next, follow the flex cable coming from your main board and it should terminate at a board the has two other flex cables on the opposite side, going to the LCD driver boards (long skinny boards to connect to the LCD via multiple ribbon cables) Do not "mess" with the driver boards. That board is the T-Con board.

Now, remove one of the ribbon cable from the T-con board and turn your TV on. Let us know what you get. Turn your TV off and unplug the power. Reconnect the ribbon cable previously disconnected. Now disconnect the second ribbon cable. Reconnect the power and turn your TV on. Let us know what you get.

Before you do all of this, check to see if your on screen menu shows properly. If it does it is your main board for sure.

Adding images to an existing question