Q: Wi-Fi issue

Please read this whole message before doing anything.

This procedure is a diagnostic test. It’s unlikely to solve your problem. Don’t be disappointed when you find that nothing has changed after you complete it.

The purpose of the test is to determine whether the problem is caused by third-party software that loads automatically at startup or login, by a peripheral device, by a font conflict, or by corruption of the file system or of certain system caches.

Disconnect all wired peripherals except those needed for the test, and remove all aftermarket expansion cards, if applicable. Start up in safe mode and log in to the account with the problem. You must hold down the shift key twice: once when you turn on the computer, and again when you log in.

Note: If FileVault is enabled, or if a firmware password is set, or if the startup volume is a software RAID, you can’t do this. Ask for further instructions.

Safe mode is much slower to start up and run than normal, with limited graphics performance, and some things won’t work at all, including sound output and Wi-Fi on certain models. The next normal startup may also be somewhat slow.

The login screen appears even if you usually login automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.

Test while in safe mode. Same problem?

After testing, restart as usual (not in safe mode) and verify that you still have the problem. Post the results of the test.

This sounds more like a bad router than anything else. Have you tested on another network?

Start up in Recovery mode. In the OS X Utilities screen, select Get Help Online. A clean copy of Safari will launch. No plugins, such as Flash, will be available. While in Recovery, you'll have no access to your saved bookmarks or passwords, so make a note of those before you begin, if they're needed for the test.

Test. After testing, restart as usual and post the results.

I'll assume that you tested long enough to be sure there really was no problem in Recovery mode.

Please test after taking each of the following steps that you haven't already tried. Stop when the problem is resolved. Back up all data before making any changes.

Step 1

Take the applicable steps in this support article. The Wireless Diagnostics program generates a large file of information about your system, which would be used by Apple Engineering in case of a support incident. Don't post the contents here.

Step 2

Disconnect all USB 3 devices. If you don't know which are USB 3, disconnect all USB devices except keyboard and mouse.

Step 3

If you're not using a wireless keyboard or trackpad, disable Bluetooth by selecting Turn Bluetooth Off from the menu with the Bluetooth icon. If you don't have that menu, open the Bluetooth preference pane in System Preferences and check the box marked Show Bluetooth in menu bar. Test. If you find that Wi-Fi works better with Bluetooth disabled, you should use the 5 GHz Wi-Fi band. Your router may not support it; in that case, you need a new router.

Step 4

Open the Network pane in System Preferences and make a note of your settings in the Wi-Fi service. It may be helpful to take screenshots of the various tabs in the preference pane. If the preference pane is locked, unlock it by clicking the padlock icon and entering your administrator password. Delete Wi-Fi from the service list on the left by selecting it and clicking the minus-sign button at the bottom. Then recreate the service by clicking the plus-sign button and following the prompts.

Step 5

Reset the System Management Controller.

Step 6

Reset the PRAM.

Step 7

Launch the Keychain Access application. Search for and delete all AirPort network password items that refer to the network. Make a note of the password first.

Step 8

Make a "Genius" appointment at an Apple Store, or go to another authorized service center.