I want to diagnose my iPhone

You can contact Apple Support using this link:--> Get Support, or make an appointment and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector

After checking it out, they'll give you all the available results/options.

Regards

Giulio

The link on top of this page under "Related Article" is the process. If that link did not help then please mention the exact difficulty faced by you.