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When did this start, How often does the connection drop?
Please ty to delete your existing Wi-Fi Preferences in El Capitan OS to Start Fresh :
5.Move all of these files into the trash (you can also make a backup of those files)
6.Reboot the Mac
7.Turn on Wi-Fi from the wireless network menu again
Let me know the results
Sure, let me know when you are available
Has everything worked out please?
Ok, I hope it works out.
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Just click on the first icon in the Dock the face the Finder then Go at the top of your screen to Go to Folder and put in
then hit Return. In the folder that opens find SystemConfiguration and drag all the items except com.apple.boot.plist
to the Trash then restart the computer and immediately hold the Shift key until you see the Apple logo. This will activate Safe boot and it will check the drive for errors among other things. It may take longer to boot and you may see a progress bar during part of the process. If it starts up OK… login then restart the normal way from the Apple Menu. You will have to reconnect to your WiFi. Let me know after.
No nothing else you need to do, let me know
1. Shut down the computer.
2. Plug in the power adapter to a power source, connecting it to the Mac if its not already connected.
3. On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time, if it starts up you held the power button before the other keys, try again.
4. Release all the keys and the power button at the same time.
5. Press the power button to turn on the computer.
6. Press and hold the Command-Option-P-R keys before the gray screen appears.
7. Hold the keys down until the computer restarts again at least twice.
8. Release the keys.
It won't hurt
It sounds like you may be back in Safe mode it should not ask for passord unless you have it set to do that at every strartup, just login and restart the normal way.
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It may be a hardware issue.
Reboot into the Recovery HD by choosing the ‘Apple‘ menu > Restart, then hold down the ‘Command’ + R’ keys on the keyboard during the start up process. If you can't get to that menu just hold the power button for 10 seconds until it shuts down. Then start it up again holding those keys. When you see the Apple Logo you can let go.
When the machine boots you may have to choose your language then you will see the OS X Utilities screen. Select the Disk Utility and then Continue. Then in Disk Utility select Macintosh HD in the left column (if there are two with that name or it is named something else choose the first indented item) and then on the right select the First Aid tab then select Run. If that is ok restart and upgrade to Sierra 10.12: http://www.apple.com/macos/sierra/
No you did that before didn't you?
OK there is no need to do it again once it is reset. If it continues to have erratic WiFi connectivity after upgrading I would say you have some kind of hardware failure maybe only a loose antenna but you would need to have it looked at.