Precautions for purchasing iPhone:

Don't turn it on after unpacking, shake it to see if there is any loud noise and loose buttons, then check if there is any paint falling off the fuselage, and finally check if there is any dust on the camera.

Boot activation (if this machine is booted directly into the phone interface, it means it is second-hand). When booting from the white apple, take some time to have a rough look at whether there are obvious bright spots or bad spots on the screen.

In the last step of activation, choose not to send error reports. After the boot is activated, first try whether the touch screen is smooth and the camera function is good. Then check the serial number and activation time of the machine (PS: If you activate it on the 28th today, it may be displayed as the 26th and 27th on the website, which is normal because the server is in the United States, and there is a time difference).

Call a number, see if the call quality is good, if there is any noise or sonic boom, and then see if the ear will be black when listening to the phone. The purpose of this trip is to test the sensor. If it doesn't turn black, there is something wrong with the sensor.

Turn off the phone and connect to WIFI, log on to a website with picture wallpaper at will, and find some black and white pictures to observe carefully. Black mainly depends on bright spots and bad spots, while white mainly depends on whether the overall tone of the screen is consistent. If it is not the same, it means that it is a screen of yin and yang. When checking the screen, remember to turn the screen to the brightest. At the same time, keep the WIFI connection, and go to several networks casually to see if the WIFI will be interrupted.

As for the "cold screen and warm screen", you really don't have to care too much, as long as the lighting point doesn't leak light and it looks comfortable.