I have a tcl 55s405tbaa That turns on and screen Stays black but has sound What part am i needing to fix this.

I'd start by testing the 12 volt rail at the timing board, see if it drops as the set goes to a black screen, if so, unhook the panel ribbon cables, if the voltage stays your panel is bad and the set isn't repairable, if it still drops off replace the tcon board

Led strips need to be repaired or replaced ,

https://www.youtube.com/watch?v=KaKi5aC9fUQ

That is usually a sign that the power supply or the LED's in the display are failing. You may be able to order new power supply or parts for it from shopjimmy.com

Chances are you have a bad main board

Have you done a flashlight into the screen test to see if there is a faint picture or not?

They dont list any LED strips and that may be a good thing: https://www.shopjimmy.com/tcl-55s403-complete-tv-repair-parts-kit-see-note.htm

https://www.shopjimmy.com/tcl-55s403-complete-tv-repair-parts-kit-see-note.htm

your on the right track but Repairs will cost as much as a new one. good luck

If you wish to get some details; visit the site linked here. Pull up older posts.
https://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.
Search the site by type in the "brand name" to your device; [Exclude the model number], in the "Search box at the top right of the display window of the Homepage" to get gathered related posts. See the site by a desktop. Mobile view has no search box. If you are viewing it by your mobile, see it in webview.]..

Do you have or get sound or is it total off?