When the motherboard makes fights
However, be aware that old devices are no different. In fact the whole range 6 (6, 6S, 6 Plus) and iPhone 7 show symptoms of weakness within the motherboard . On 6 and 6S the chips that manage backlight and GSM modem can fail. On 6 Plus it's up to the touch chip, which in many cases causes an intermittent gray band to appear at the top of the display. On 7, however, it is the turn of the audio chip.
How does Apple respond? Simply advising you to buy a new iPhone. These are problems that can be easily solved but that the manufacturer would cost too much, which is why they direct customers to purchase a newer model.
What they don't tell you, however, is that you could (and should) go to a specialized service center. If in the United States they solve a bit of everything with class action, in Italy there is SOS Phone . Bring us the offending iPhone and we'll take care of repairing the motherboard at a super affordable price. Seeing is believing.