Q: iMAC 21" WIFI failing after 10.7.4 update

And this works for me: changed the router chanel from 9 to 11 and wireless mode setting to 802.11 n/g/b mode.

The Mac then automaticaly chose 802.11n mode. This setting works so far well. Testing it with listening to www live radio. No interruption so far.

My old "problem" setting was:

Hope this works for you too. Best regards Markus

Maybe I'll give Mountain Lion a try. I've had a stable wifi connection for a few weeks with the Asus USB wireless adapter I bought, but would much rather prefer to use my imac's internal one instead.

The Fix for Airport disconnect problem on 10.7.4:

1.  In Finder, go to GO and scroll to and select Computer(s)

2.  Double click on Hard Drive

3.  Double click on Library

4.  Double click on Preferences

5.  Double click on System Configuration

6.  Drag the following files to the Trash, they will reload upon Restart

          com.apple.airport.preferences.plist

          Networkinterfaces.plist

          com.apple.network.identification.plist

7.  Restart

8.  Go to wireless icon (top right menu) and select your network

9.  Once connected, manually put computer to sleep (apple menu) and wait 10 seconds

10. Wake up cvomputer and if still connected, problem solved ... so far.

Hope this helps.

I had the same problem but rebooting didn't help.  I totally lost all wifi connection, even though it showed that I was connected to wifi. 

I have a MacBook Pro and an Airport Extreme and downloaded the 10.7.4 update a few days ago.  Initially, my wifi worked, then on the second day it started to work only off and on.  By the third day, I had nothing.  So I called Apple Support and spent an hour and 40 mins as they walked me through all these different diagnostic tests.  After speaking to three different people, they concluded that it was a hardware problem and wanted me to take my MacBook Pro into a store for a diagnostic test. 

After I got off the phone with them I thought maybe it's just a software problem, since my wifi was working fine until I downloaded that update.  So I decided to upgrade to the Mountain Lion, OS X 10.8, and now my wifi is working fine!  Been on the internet for about an hour now and no drops.  Totally worth the $20 I spent!

This worked for me (so it seems):

Copy&paste from another post:

I have been writing whenever I can to encourage people to try the following solution that was posted elsewhere in the Apple discussions.

Go to: Library/Preferences and delete the SystemConfiguration folder. Reboot. The system will remake this folder. Evidently, some file in this folder gets hosed and needs to be remade.

I saw this fix in someone's post on May 25, 2012. I tried it and it worked immediately for three Macs I have. It has been working great for three days now. On May 27, I installed 10.7.4. The fix is still working. I had been experiencing severe wifi network drops requiring the usual procedure of turn off wifi, turn it on, then connect to the router. Now I dont' do anything. It works after reboots, after logouts and then logins, swithes to other users and back, etc.

There is nothing to lose by trying this. Hopefully it will work for some of you experiencing this problem.

Please let us all know your experience with this fix.

Tried it some time ago.  Did not work for me.  Some fixes like yours work for some but not all iMacs.  Seems rather hit and miss and quite baffling.

OS 10.7 gave me no problems then I ugraded to Mountain Lion OS 10.8.   Problems with iPhoto now resolved, but wifi drop out a real problem.

Not being too technically minded I eventually went to System Preferences, then Network and reset yet again the wifi.   Then I clicked the lock whilst all was still working and no drops outs since.   This may be a fluke having read all your sumissions, but it seems to go.  

Apple should be ashamed that they have not resolved this issue and force people (customers) into these convoluted work arounds.

Well fix worked for about a week then back to the old problems of dropping wifi on waking from sleep.  Apple tech has no other options, recommending replacing wifi and blue tooth units.  Even though I am under warranty I don't think this is resolution as no problem until Mountain lion.  Even have done a complete reinstall of Mountain lion and restore from time machine since.

I have the same issue with Mountain Lion

Unbelievable.. when they finaly resolve this "basic" feature..

As someone who has lived with this problem for almost a year I can finally say that the upgrade to Mountain Lion has addressed the problem on my 21" iMac. Been running now for 2 weeks and not a single drop. Seamless upgrade and I would advise doing it rather than trying to find a workaround.

Same issue here at Mountain Lion. But now the Wifi doesn't simply drop, but the hole iMac freezes. Setting the Access Point to a fixed channel fixed the problem...

We have seen this problem with several clients after upgrading to 10.7.4 and found that the issue was due to the security setting for the particular "remembered" wireless network being mysteriously changed to None instead of  WPA2 - the setting before the upgrade.

I have solved my wifi issue where the connections keeps dropping intermittently with my imac.

1st My Router - DIR 655 - Changed the WIFI setting from 802.11n to 802.11g/b as seen below in screen shot.

Reason: I read somewhere that on some routers 802.11n being broadcast @ 2.4GHz is something that might cause a problem. I still have my encryption set @ WPA2/AES.

2nd - Renaming the following directory & Rebooting your Imac

"/Library/Preferences/SystemConfiguration" to

"/Library/Preferences/old-SystemConfiguration"

and reboot. This should wipe out old preferences of wifi

3rd - Reconfiguring your WIFI connection

Reconfigure you wifi connection in imac and in addition go to the network preferences and update the MTU to 1430 as seen below (System Preferences -> Network -> [Select] WIFI  --> ADVANCED --> HARDWARE tab)

Now while holding the Option move your mouse and click the WIFI and you should be able to see the details of the connection which should now show the imac connected to the 802.11g

The above steps helped me solve the WIFI issue and have not seen a problem since. Hope this helps you all. Good Luck!

Thank you for this tip and instructions ~ saved my bacon. I have an external drive that is a bootable clone of my 17" MacBook Pro internal drive. I booted up from the external drive (for some program verification) and it loaded but when I clicked on a Calendar alert it froze. So I re-booted to the external drive once again and it just hung about 2/3 into the boot process. YIKES!

I first ran "Disk Utility" and then DiskWarrior to no avail. So I re-booted into single user/verbose mode with the external drive and saw it was hanging at "switchvap from 1 to 1" and the text before that had to do with wireless protocols saying something along the lines of "is 0 but should be 16" multiple times.

So I searched for "switchvap from 1 to 1" and came across this link. I only had the first two files to delete (com.apple.network.identification.plist was not in the folder). I decided to just delete the two files and nothing else. I backed up the two files to the main hard drive and emptied the trash. Re-booted to the external drive and BINGO! It re-booted!

Thank you once again!

*** Let me add that this fix was used on OSX v10.11.5 (or was v10.11.6 already ? ~ I updated the external with Security Update 2016-001 before noting what version it was at ).