I Have a 42LE5400, The start up screen LG LifesGood does not turn off and go the television mode, is it a power supply or Main Board

Fault to the main power supply regulator section. Either it will be a separate board, or the part of main board. Check and replace damaged component/s at its main power regulator [SMPS] section circuit. Contact any service technician. If you wish to get some details; check the site linked here. Pull up older posts.
http://electro-medical.blogspot.com/

It will be best to replace the power supply regulator board as card basis rather than making "Surgery" to it; which usually will make loss for both money and time. Even though you can make it right, it will cost around an amount equal to the cost of a new board; including the cost of (spare parts + service charge) More to say, most of the spare parts to it will be difficult to get from open electronic spare part market. This is true to most of the flat panel TV power supply boards; irrespective of its brand and screen size.

Back - light problem. 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://electro-medical.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.

Just a guess (an iterative process)... but

Main board... EAX61532702
You MUST verify these numbers yourself.

If you are inclined to tackle this yourself... take a look at my 2 previous postings ... but I only GUARANTEE if you go in carelessly... you WILL turn your device into a pile of GARBAGE (good only for parts).

I was completely FLOORED to find reasonable prices at SEARS... (guaranteed fit, (CALL 1-800-683-1693)).

Shop-Jimmy repair/return has kits(check out below).

Always use an ANTI-STATIC WRIST STRAP...Work safely (real pros REMOVE power (when possible))...

Good luck...

42le5400 main board EAX61532702

Part Search Results

LG 42LE5400 UC AUSDLNR EAX61532702 0 Main Board

possible t-con board failure

It's easier to replace the main board

Try a hard reset to this set first. If it do not help you, the main board to this set is damaged. Contact any service technician; if this is the case.
To make it a hard reset, just unplug the TV from AC mains wall socket, and re-plug it back after 15 minutes or so. If you do this by overnight; will be better.
If you wish to get some details; check the site linked here. Pull up older posts. Surf the site with patience. http://electronicshelponline.blogspot.com/

Probably not main board but a short on the y- sustain board or a bad power supply

The fault is with its main power regulator board. It will be better to replace this board as card basis. Contact the authorized service center for LG, to buy a genuine board. For more information go to http://electronicshelponline.blogspot.com/

The problem lies either in the power supply or the main board. But most probably it lies in the main board. These boards communicate in switching on the television sequentially. The power supply sends a signal to the main board when AC power is supplied and puts the television on stand-by. When the TV is turned on, the main board sends a switched command to the power supply to deliver it's output for audio and video processing. At the same time it turns on a supply voltage for the backlights.

What probably happened is, the signal sent by the main board to the power supply is being switched on intermittently and causing the turning on and off of your television.

1. Remove the back cover of the television to check the power supply and the main board. The power supply is the one with transformers on it and located at the center while the main board is the one where inputs are being connected to and located at the lower right back of the television.
2. Check the power supply for any swollen capacitor.
3. Turn on the television and check if the LED on the main board flashes as the TV turns on and off.

Please inform me what will you find above and if you can do electronic repairs. Just tell me if you need guidance in replacing parts for your TV.

The problem can be in the bcaklight inverter board or the main power supply is not supplying the power to the inverter board. The main power supply gets the turn on signal from the control/main board.