Tv sony stop after 20 min standby led flashes 3 times

There are particular blinks meaning on sony TV. I can give you the problem from your sony tv if you specify or count how times the LED flashed.

In standby mode (red led on/after the led stops from blinking), press "Display", "5", Volume (-)", "Power on" on the remote control in sequence, self diagnostic screen will be display, let me know the what is on the screen to diagnose where the fault came from.

Sony TV LED & Blinking Codes - What Does It Mean When Standby/Timer Light Flashes Sony has provided a "self-diagnosis function" with your TV, that in the case of abnormalities or problems the LED or red blinking light on the Timer/Standby indicator flashes a number of times to indicate the issue.
when there is trouble with your Sony Television set -- it indicates the issue in either the diagnostic mode, or even more commonly by flashing a series of LED blinks or flashes on the front of your television. Each set or frequency of the blinks indicates a very specific problem with the television itself.
Sorry but unless you may be able to repair the television set yourself , please call a local repair shop to the have a repairman sent out,,

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you probably need new power supply diodes or solder connections, you need a qualified tv repairman to fix this.

My TV (Sony Bravia LCD KLV-32S310A) goes of after 20 seconds when it is switched on. The standby indicator then flashes (red) five (5) times then there is a two seconds break followed by another six flashes then two seconds break etc. What might have caaused this and what should i do?

Sony TVs generate an error code. The front panel LED (red) will flash anywhere from one to eight times which can isolate the problem circuit. It is not always accurate.

If you have a test meter which can read DC voltages and resistance in ohms, you can locate the problem area. If you lack the expertice to do so, contact a sony authorized servicer. If you have the ability, start by checking fuses on the "G" board ( power supply board) and the "D" board or deflection board.

A major problem area in this TV is the convergence circuit. There are six fuses around the convergence output ICs on the "D" board.

If any are open, replaces the output ICs.

4 LED flashes indicated a faulty IC board

Flyback is defective on the D board. You can get a new replacement flyback with free instructions at the following link, or they also have the new replacement D board for your model also. Heres the link, just click on the Sony parts category.
http://servicemanuals.vstore.ca/

When the set ends up flashing the LED, it should be a repeatable count. Can you please report that count here? Also, since the set eventually works, I'd suspect a bad connection or a part that has changed value over time. The blink code will help to narrow down the offending circuit.
Dan

3 blinks means lamp cover error