How do I run a diagnostic test on my iPhone?

You can have Apple perform diagnostics on your iPhone. It can be done in person at an Apple Retail Store or at an Apple Authorized Service Provider. You can also contact Apple Support and request remote diagnostics.

If you’re in the US please call  1-800-MY-APPLE or outside the US, consult the support articles below.

Contact Apple for support and service - Apple Support

Genius Bar Reservation and Apple Support Options - Apple

Explore Service Locations

Official Apple Support

Last time I this this is limited to USA

How to put your iPhone in diagnostics mode - Apple Support