My LG G3 will not turn on with the power button alone. It will turn on by holding the power button down and then plugging...

I swapped the battery with another G3. No difference on my phone or his. Mine still did not start. His worked just as before. I had no recent updates on the phone before the problem started. No drops or liquid and of course the phone is about 45 days out of warranty. I've ordered a replacement switch and cable. They should be here within 1 to 2 weeks. I'll post the results. Thanks again for your ideas.

That is very good reasoning. Unfortunately I explained the phones starting after connecting it to a charger badly. If I have the LG G3 plugged into a charger and then press the power button nothing happens at all. If I press and continue to hold in the power button while I plug the phone into the charger it will start up provided I then disconnect the charger as soon as I see the LG splash screen. If I hold the power button on the phone past the the splash screen the phone just goes into charging mode. It is the strangest thing I've ever seen. Once started the phone can not be turned off by the power button, the battery has to be pulled. Not great for the phone. Both volume up and down work fine. Thanks again.