Liquid detected in lightning port

Contained in that link is the following:

If your iPhone and Lightning accessory aren't wet

If you see this alert every time you connect a certain cable or accessory, that cable or accessory might be damaged. Contact the manufacturer.

If you see this alert every time you connect an Apple Lightning cable or accessory, contact Apple Support.

Since this ONLY seems to happen when you plug your phone into your car system, I'd try a different cable and if that doesn't help, I'd click the blue link to contact Apple Support for help or I'd schedule an appointment at my Apple Store Genius Bar for evaluation.

Ps: I've connected my iPhone to my car system by cable after the update to iOS 16.5.1 with no such warning, so it's most unlikely this was caused by the update to your iPhone. We're also not seeing other people suggest the update may have caused this issue to their iPhone.

Did you Force Restart your iPhone after you updated it? This is something I recommend you do after every update. To Force Restart your iPhone:

1. Press and quickly release the volume up button
2. Then immediately press and release the volume down button
3. Then press and KEEP HOLD the side power button and keep holding it past the swipe to power off and keep holding until the screen goes dark and keep holding it until the Apple Logo appears, then let go. Sign back in when prompted and test your phone in your car.

If you see one of these alerts, your iPhone has detected liquid in the Lightning port or on the Lightning cable or Lightning accessory.

If you see a liquid-detection alert on your iPhone

Then you'll need to schedule an appointment at your Apple Store Genius Bar and show the phone to Apple.

Let us know the outcome if you can, thanks.

You're welcome. I'm sure they'll get this sorted for you.

The hard reboot fixed the message for my iPhone 15 pro. The cable is a cheap one, but works ok for my wife’s iPhone 12, and for my phone post reboot.