Though Apple iPhone is a great smartphone, there are some common problems associated with all versions of the iPhone including – iPhone 5, iPhone 6, iPhone 7, iPhone 8, iPhone X. iPhone Bluetooth Not Working is a very common problem
If your iPhone Bluetooth is Not Working then you may have to do one of the following:
1. Update the iOS Version
Bluetooth not working is basically a software related problem and not any hardware related issue. The problem is mainly related with iOS 11. To solve the problem, all you need to do is update the iOS to the latest version. Apple team does all the bug fixing in the iOS Updates. To Update your iPhone wirelessly, follow these steps:

Plugin your iPhone into power source.
Connect to the Internet with Wi-Fi.
Got to -> Settings -> General -> Software Update.
Tap Download and Install.
Follow message to temporarily remove apps to create more space for the update.
Tap Continue.
Later, iOS will reinstall these apps.
To update now, tap Install.
You also get the option to Install Tonight or Remind Me Later.
Your iPhone may ask to enter your Passcode.

2. Reboot iPhone
If the Bluetooth problem is not fixed even after updating the iOS, the follow these steps:

Go to Settings -> Bluetooth -> Toggle OFF Bluetooth
Reboot your iPhone
Turn ON Bluetooth and Check Connectivity


Pair with another Bluetooth Device
If your iPhone Bluetooth Problem is still not solved and you are not able to pair with a specific Bluetooth device, the do following:

Forget the unpaired device.
Pair with another Bluetooth device. If you can successfully pair to another device then you should forget the device that is not able o pair.
Go to Settings -> Bluetooth -> Click the little “i” icon next to the device giving problem -> Click “forget this device”.
Find the device again, and try pairing.

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https://itwantech..com/2019/06/iphone-bluetooth-not-working-problem.html