I took my iPhone 6 into the apple store because the volume buttons were not responsive. They were clicking mechanically but would get stuck in an on or off state. There was also an intermittent problem with the touch screen. Apple said the phone had water damage and so they would not repair it and offered a $300 replacement.
As a last-ditch effort I started a repair as if I needed to replace the buttons. Then I used a 10x jewelers loupe to inspect the button terminal contacts. I saw some corrosion on one of the terminals. I cleaned it with a q-tip that was lightly wetted with 70% alcohol. I then started inspecting the remaining terminals. I noticed a bit of dirt in one of the screen connector contacts that I also removed with the q-tip.
I powered the phone back up and it is working great! All it cost me was a q-tip and a couple hours of my time.