Backlight out on vizio E550i-B2 do i replace the led driver or power board or both. there IS sound and you can see the picture with the flashlight test?

This model set has local dimming and not a lot of issues with backlights failing, but it has issues with connector CN202 having bad solder joints from the factory. Chances are there isn't anything wrong with your old board other than bad solder joints and I have a feeling the replacement you got has the same issues but only a couple of them are bad making most the lamps light. If you've able to solder and have an iron you can repair it yourself, a little free advice is use a lot of flux to keep the connectors from wanting to solder together as they're quite close to each other.

This model set had some issues with the backlight driver connector solder joints, make sure those are ok first, it they're ok it's a backlight issue

See video here:
E550i B2 blacked out screen VIZIO NO PICTURE HAS SOUND FLASHLIGHT TEST

Hi,
This model develops bad solder joints on the connector between the power supply and the led driver board and loses backlighting. It's hard to believe, but the whole connection goes bad. All those pins supply only 24v and gnd. Resolder all of them and the tv will work.
It's the connector between that large yellow board (p/s) and the center green board (led driver).

Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis. http://electronicshelponline.blogspot.com/ To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced. To some models of sets, all these section circuit will be integrated at its main board. If this is the case, you have to replace the main board.

If you can see an image with a flashlight and you can control the tv with the remote then the main board is not the problem. You've replaced the LED driver board, so that leaves the LED strips inside the LCD panel as the likely problem.

The led is what makes the picture try replacing that it it's the only one you didn't change out

What is the part number that you are replacing each time

One or more of the LED backlight strips behind the LCD have failed. Difficult and sometimes expensive to fix.

U are on the right track - problem is caused by the board that drives the backlighting - the inverter board.

If you can get the number for the board it may be available on the cheap on ebay.