Bluetooth is not working on iPhone 15 Pro Max after iOS 18.4.1 update

Perhaps a long shot, but a good starting point:

Do you have a 3rd party VPN installed, even if you are not using it? If so, delete the VPN profile in Settings/General/VPN & Device Management/VPN. Don't just turn it off; delete the profile. If that fixes it, you can try reinstalling VPN. 

BTW, Apple is not here. This is a user-to-user technical support site. And, with 2 billion iPhones in use, no matter what problem you have, there are guaranteed to be others with the same or a similar issue. apple will look for a cause if thousands of users report the same problem by contacting support (NOT by posting in this forum), so the next step for you is to contact Apple Support (NOT Best Buy).

Best way: 

• Install the Apple Support app on your iPhone or iPad - you can contact support directly from the app, and the tech will have (with your permission) access to diagnostic data on your device

Other options:

• • Click on “Support” at the top or bottom of this page
  • Go to Contact Official Apple Support

(Note that both of the above have options to receive a callback or chat)

• • Call Apple Support; the phone numbers are in this link→Contact Apple for support and service - Apple Support
  • Make a Genius Bar appointment at an Apple Store→Genius Bar Reservation and Apple Support Options - Apple
  • Find an Apple store or Apple partner service provider→Find Locations

"Bluetooth is not working on iPhone 15 Pro Max after iOS 18.4.1 update"

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Report this Update Bug to Apple...

So, do your Part: Provide Apple with feedback on this, letting them know about this issue.  Apple may not get back to you directly, but the more feedback they receive on this, the more they will know what is occurring, and what bug fixes to include in updates to come. Apple will know nothing of this, if you don't report it to them.

1. Go Here: Feedback - iPhone - Apple
2. Select: "Bug Report" for the "Feedback Type"
3. Comment: that this all began after installing an update. Include the URL of this thread
4. Proceed from there as necessary

I have a Kenwood tuner in my car. Everything worked fine until the 18th for update including the bug fix update on 18 for I can work if I only hook up my cable it will not work by Bluetooth or wireless.

Remove all Bluetooth conections previously established and create new connection for your Bluetooth device. Worked for me.