Greetings mw_123,
It looks like you joined the Apple Support Communities from the correct article that offers information and support for your issue: iPhone parts and service history - Apple Support.
An Unknown Part 
message appears if the part installation is incomplete or if the part:
• Was replaced with a nongenuine part
• Was already used or installed in another iPhone
• Isn't functioning as expected
Also, be sure to review the bottom section of that page for more information about why this issue may appear.
Importance of service by trained technicians using genuine Apple parts
For most customers, visiting a professional repair provider with certified technicians who use genuine Apple parts is the safest and most reliable way to get a repair. These providers include Apple and Apple Authorized Service Providers, and Independent Repair Providers, who have access to genuine Apple parts.* Repairs performed by untrained individuals using nongenuine parts might affect the safety and functionality of the device. Genuine Apple parts are designed to fit precisely within the device. Additionally, repairs that don't properly replace screws or cowlings might leave behind loose parts that could damage the battery, cause overheating, or result in injury.
Depending on your location, you can get your iPhone serviced — in or out of warranty — by visiting an Apple Store or Apple Authorized Service Provider, or by shipping your iPhone to an Apple Repair Center. Genuine Apple parts are also available for out-of-warranty repairs from Independent Repair Providers or through Self Service Repair.
If you have further questions about your warranty and coverage, look here: Legal - Hardware Warranties - Apple. Or, you can contact Apple directly. You can reach out to them by clicking here: Contact - Official Apple Support or over the phone by calling the number that is most local to your region: Contact Apple for support and service - Apple Support.
Kind regards!
