My Vizio TV has dark spot top left corner

It is possible that the issue you are experiencing with your Vizio M55Q6M TV could be caused by a problem with the backlight. The backlight is responsible for illuminating the display on the TV, so if it is not functioning properly it can cause uneven lighting or areas of the screen to appear darker than others.
Other potential causes of the issue you are describing could include a problem with the display panel itself, or an issue with the TV's software or firmware.

If you wish to get some details; visit the site linked here. Pull up older posts.
https://electronicshelponline.blogspot.com/
Line faults & possible causes to LCD & Plasma screen are given with illustrations./

Replace the LED back lights strips.

can you attatch a picture ?

If you wish to get some details; check the site linked here. Pull up older posts.
http://electronicshelponline.blogspot.com/
Faults & possible causes to LCD & Plasma screen are given with illustrations. You can get an idea about the fault to an extent. Faults to all LCD & Plasma screen are similar, irrespective of its brand and screen size.

I have the same TV - I have a dark spot in the upper left hand corner. I have always thought this TV was dark. Be sure to get OME lamp. Cheaper ones are not worth it and can damage your TV more.
dlplampsource,com--Just a note if this ever happens to you. TV would not come on in the AM-Had no picture, but had sound. It was not the lamp, but the safety switch. It's blue and at the bottom of the lamp housing. As soon as I flipped it and replaced the back cover, the TV came on.

If half screen is affected it will be either a board (LCD and Plasma have different names) but in those one board drives each side of the screen unit--worst case is the display panel itself which is fatal from a cost point of view.

The inverter board is common on this set to go out, once dark then you can use a flashlight to determine that it is bad, 2 in away from screen and you should see the picture in background, the sound is something else, it will be located on the main video board, look for bad caps bulging out.

Not likely a bulb problem.

The model is a LN-S4041D. A 40 inch LCD. I can send you a photo but I can't get the file size small enough to post