LG C5 OLED evo TV picture is too dark or keeps dimming

The checks in more detail

Turn the ambient light sensor, Auto Brightness, off for a test, since a sensor covered by a soundbar or reacting to a dark room dims the picture. If the TV is in a mode for HDR content and the picture looks dim, the HDR settings need attention: HDR on an OLED looks dim in a bright room compared with a LED TV, and the Dynamic Tone Mapping option brightens HDR scenes. Source issues matter: a console or box using limited black levels or a wrong HDR setting produces a dark or dull picture, so check the source's HDR output and the black level settings, Auto in most cases.

Check the HDMI Ultra HD Deep Color setting for the port in Settings, General, Devices, HDMI Settings. Remember that OLED screens protect themselves with automatic brightness limiting, which reduces brightness for a bright, full-screen scene, so a white screen looks dimmer than a small bright highlight; this is normal. Check the room: a bright window or lamp makes a screen look dark by comparison, so test in a dim room. Update webOS. If the picture is dark in the menus too, with every setting at default and energy saving off, the panel may be aging or faulty, and LG support should look under warranty.

A dim band or uneven brightness needs checking too. Do not turn the brightness to maximum for long periods. Photograph the screen in the dark with the TV's brightness at its midpoint to see whether there is uneven dimming on the edges or in corners. A uniform dim picture is a setting; a patchy one may be the panel. Note whether it improves a few minutes after turning on, because OLED panels reach full brightness after a short warm-up, and compare using the same scene each time so that you judge fairly.