My Vizio e500i-b1 LEDtv went out last night. How can i tell if it's the main board or the tcon board that isn't functioning?

You can change boards in your TV all day and it's most likely no going to fix this set. Have you ever had a set of miniature Christmas lights that seemed to work great then all a sudden none would light, either because a bulb burned out or fell out? There are 2 types of electrical circuit wirings, one is series the other parallel. In series like your LED backlights are wired in your TV there are about 8 button lamps per circuit that have a 24 volt DC supply serving each set as this TV has what's called local dimming, one of the lamps has opened up making that set not light up, the power supply has over voltage and over current protection for each supply if one lamp opens up they all go out and this is exactly what's happened in your TV, the power supply safety circuit is non discretional, if it detects an error it shuts down the entire backlight circuit giving you no backlights. As far as a break down on what fails 5% of the times it's the main board 5% its the power board and the other 90% of no backlight failures on this model TV comes from the backlights being bad. You can change just the one bad strip if you wish but it's advised to change them all. Good luck!

You might want to check the VIZIO website for help. Given how cheap even 4K UHD TV's are today, probably not worth fixing again. Unless under warranty, of course.

Generally if you can change the channels and still hear the sound then it will be the Tcon board.

hi there, can you explain why you replaced the mainboard and psu ?

Yes go to SHOPJIMMY.com they will have vids on how to test and all the parts you may need. Good Luck

Fault at its main digital board. Short circuited component/s at it. Contact any service technician; or the authorized service center. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. http://electronicshelponline.blogspot.com/ In most cases, the main board might have to be replaced. If so, a software [firmware] update too might be necessary.

Vizio TV No Image Black Blank Screen No Backlights Basic LED... Led drivers are on the power supply board. Here's a video

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.

Have just looked on you tube , I typed in fix for VIZIO E500i -B1 power off and a 2.24 minute video appeared . if that does not solve your problem type the product info and fault again and check the videos that pop up . hope this info is of use to you....