One of the most common issues that are found in Apple iPhone 6 is its power button problem. The reason for this problem could be anything. This could either be due to water damage or just the basic wear and tear. In the case of basic wear and tear, power button losses its Springiness hence you need to press it harder than usual for it to make it work for you or it could either fail completely.
So if your iPhone 6 power button is not working to repair it you just need to replace it with a new one. If the prints of the button are damaged then you are going to have to make the jumpers as shown in the diagram.
Hardware Solution for iPhone 6 On/Off switch
The first thing you need to test is there any water damage or dust.
if any thing just clean it with the help of IPA or any cleaning liquid.
If same problem then check the Track ways of iPhone 6 power on/off switch. Marked in below image.
If any of the track is missing make a jumper with the help of jumper wire.
Solution By buds_guy
Hints for repair: when the device will unable to power up when hitting the power key button but it shows charging indication when you plug-in the battery charger, the problem may possibly lies on the power key line paths. But before tracing the line paths, you must see to it that the keypad membrane is perfectly clean and free from dirt, corrosion and oxidation. You can check the power ON/OFF voltage to the inner core of the switch key terminal pads, this voltage may range approximately from 1to 3 volts. If there is no voltage reading you get on that spot, locate and trace the test spots for the power switch, see the picture guide above, it is located just near the keypad LED. Make a continuity check by using a multimeter to those corresponding spots indicated on the picture guide, from the power key pads through to each corresponding component.
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