The main board (Panasonic calls it the "A board") has detected a fault in the power to the LCD panel. Most likely the 12 Volt section of the power supply (P board) has failed, but the problem could be the main board or LCD panel. The troubleshooting procedure can be found in the technical manual for this series (link below).
There's a good chance the problem is burned-out electolytic capacitors on the power supply board; these are often identifiable visually. Good capacitors have a flat top, but burned-out units will have the top domed out by internal overpressure. See the photo for examples. The capacitor in front and the one to the right with the glue on it are good; the two marked capacitors are bad. You can unplug the TV and look at the power supply (board on the center left with a cable going to the power cord socket). (Do not touch the board for at least ten minutes after the TV is unplugged; the main input capacitor can retain a high voltage charge for some time.) If you remove the power supply for service, only handle it by the edges, and do not put it in or on a plastic bag unless you know the bag is anti-static. Static electricity can destroy the chips and transistors on the board, even small charges on your fingers that you don't feel.
Replace bad capacitors with capacitors having the same uF (micro-Farad) size, the same or higher voltage rating, the same or better temperature rating, and high ripple current and lifetime ratings. I strongly recommend using the Panasonic FR series capacitors for this application. If a higher voltage rated capacitor fits in the space (including vertical space), spend a few extra pennies on it because it will generally have a higher ripple current rating and longer life. Note: the polarity stripe on the new capacitor must go in the same way as the original; putting the capacitor in backwards will destroy it as soon as power comes on. Replacement requires electronic soldering skills for static-sensitive circuits.
Panasonic TC L42D2 Technical Manual Table Of Contents
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