Q: Wifi ISP keeps failing after the last El Capitan update

Hello linaandr,

Thank you for using the Apple Support Communities. I know having an issue with Wi-Fi on your MacBook Air is not what you expect. If I understand correctly, you are unable to maintain a connection to your Wi-Fi network, even if you use the Wireless Diagnostics to fix it. Because other devices on your network are not having issues, we can narrow this down to your MacBook Air. Here are the steps I suggest to resolve this:

1. First, try running a quick test with the built in Disk Utility tool, as this can correct simple issues like you're experiencing. Then restart your MacBook Air.

Disk Utility (El Capitan): Repair a disk
How to turn on or turn off your Mac

2. Use Safe Mode to start up your Mac so that it performs certain checks, delete system cache files, verifies your startup disk, and attempts to repair directory issues if needed.

Try safe mode if your Mac doesn't finish starting up
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3. You can figure out if unexpected behavior is related to user file or setting by trying to reproduce the issue from another user account. This process includes creating a new user account, logging in to it, and testing for the issue.

How to test an issue in another user account on your Mac

Cheers!