To check which LED(s) has failed, it's best to use a multimeter on diode setting, which gives just enough volts to light up the LED without dazzling. Either side of each LED are two exposed 2mm diameter test points. Put the positive lead by the red arrow and the negative by the blue arrow and the LED should glow slightly if OK

If you put the leads on the wrong way round, the LED will not light up. Note next to each LED is the diode symbol which shows the direction of current ie positive to negative, so you know which way round the leads should go.

Note: the diodes at the ends have only one adjacent test point, so the other multimeter lead goes onto the test points by the connector (either +ive or -ive to make current flow in the right direction).

Test each LED and you should find one or more does not light up. You may also find that the failed one(s) is a slightly different colour, but it is not always the case. If they all light up, then either the power supply is faulty or there is a dirty connection between the LED strips and the connector (clean them eg with a pencil rubber (eraser).

An LED normally fails 'open circuit', which is why the whole string goes out for just one failure. If the LED fails 'closed circuit' There is a dull spot on the TV and the other LED's take too much current reducing their life until another fails. The working voltage across each LED should be about 3v.

The early LED failure in this TV may have been due to the fact that one of the LED strips was not in the right place. There are guide pips (yellow arrows) showing where it should be positioned. Note the strip is on one of the pips so it can't sit flat on the metal chassis which also acts as a heat sink i.e. the adjacent LED's would run hot.

the second image shows a gap between strip and the metal surface, hindering cooling. In my case it was one of the three LED's with compromised cooling that failed.

If you don't have a multimeter (or LED tester), there some YouTube videos showing how to use a battery eg use a 9v battery and test three LED's at a time (3x3v=9v)