TC-P42S30 How do I install a new txnsn1pkuu SN board?

Unfortunately there isn't a way to say exactly what board is causing the issue, plasma sets are a lot harder to repair as there are at least double the amount of boards in them and several boards cause similar issues.

Substitute Stand Base Panasonic TC P42C1 TC P42C2 TC P42S1 TC P42S30 TC... Visit this site to get more details.
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If you have replaced all the boards, including its power and main board, some critical adjustments should be done. The TV wont switch ON, without these adjustments. Contact any service technician. If you wish to get some details; check the site linked here. Viewing it in "Mosaic" will make surf easy.
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verify that both fans are spinning when you try to power up the unit

you must resolder all the colder joint with a solder. that the first step.
However if the problem remain, you must test some components such:capacitos+resistor+diode+transisit+I.C for faulty and replace.
test any fuse or c.breaker in the board.
God bless you

Main digital board should be troubleshooted. If you are not familiar with electronics, do not attempt to repair. Whether you suffer fatal electrical shock! Instead, contact any experienced service technician, or the nearest service center