iMac Freezes after a couple of minutes of operation (but although unresponsive seems to be operational)

[tassos]

Any ideas why my iMac (mid 2011) freezes after a minute (up to 3 minutes) of operation?

From what I can understand iMac is not freezes completely as the tasks that are open, run normally!

What happens is that the Bluetooth is disconnected (the Bluetooth icon appears with a zig-zag line) and at the same time It seems that input commands are not received from iMac, neither from my wired keyboard nor from wired mouse! After that my only option is to shut it down from power button and try again.

Note however that the CPU and Memory graphs are refreshed and seem to working fine!! (see video)

The first though was of course software, disk etc. but the same happens when I try to boot in Safe Mode, even in Recovery mode.
I have managed to run a disk test in both disks (one SSD and one HHD that can boot individually) with no findings.

So, I guess the OS installation is not the issue nor an application or setting.
During my tries, a couple of times, there is a really strange noise and the OS is not loaded and also ones, when I tried to boot on "Recovery" I had an erraticall movement during selection (video attached for both)
Both issues are never appeared in past

Do you have any ideas of what causes this problem? Probably I will avoid to visit a repair shop but I am really curious what might happened, also to know what to hear and expect from the repair guy! I believe that he will also be puzzled about it.

I have attached a video that shows the CPU activity after the "freezing". The CPU seems to work normally but there is a blank gap of few seconds after freezing. (I noticed a higher activity at start with probably because the iMac still loads the OS and two small "gaps" of no CPU activity before freeze but I don't know what to look or what kind of report to generate to check this activity)

Also a video with the strange noise and another one with the crazy movement of selections in recovery with no mouse of keyboard attached!

I will appreciate any ideas!

Regards

 

[TwoH]

To me this could be an issue with either the HDD (if you have one) OR the more notorious offender - the Bluetooth card itself inside the iMac. I've seen this issue once, where it even affected wired keyboard + mice.

I'd try disconnecting the card from the machine entirely and seeing if the issue persists.

 

[tassos]

At least someone has seen this issue again
For the HDD, I don't think so! In iMac I have one SSD and one HDD, each one can be booted with different OS.
I have disassembled many times PCs, phones and televisions for fix and experiment but I never touched my iMac I don't think I have the guts to do it now but your comment if very interesting and I will suggest it to the repair guy.
Before send it over there, is any tool or report that I can use to check the status of the bluetooth card?

 

[TwoH]

I'm unsure if even the ASD for this iMac will come up with an error for the BT card, if it is the issue.

Possibly in console? I can't recall using any tool or software to diagnose the issue - it turned more into a "swap parts out and hope for the best" type thing.

 

[tassos]

hmmm... searching a bit further, I see that the Bluetooth card is located in the easiest position for removal. The only think I need to do is to remove the screen (according to this:

)
At this level I can give it a try.

If I remove completely the Bluetooth card and turn on the iMac, is it going to work? I mean the iMac. Obviously there will be no Bluetooth but theoretically if the cause of this behavior is the Bluetooth card, it will not appear if the card is not installed.
Am I thinking correctly?

 

[TwoH]

Yep, the card is very simple to remove, and yes the iMac will turn on with no issue without the bluetooth card in the machine. Just be wary on the cables connecting to the LCD and backlight when taking it apart.

 

[tassos]

Thank you very much for your assistance buddy. Really appreciate it.
I will think of it and probably I will give it a try (as I never wanted to touch my iMac).
In any case i will post an update to confirm what happened in order future users with the same issue to know what to do.

 

[benji888]

Try running hardware diagnostics before opening it up...just hold the D key when starting up.
[automerge]1595340632[/automerge]
Sorry that’s for 2013 or later Macs, check this out: https://support.apple.com/en-us/HT201257

 

[tassos]

Thank you Benji. I didn't knew this option. I hope that I will not need it in the future but I will have it in mind!

To be short...I tried it but unfortunately the pc freezed completely after almost an hour. I waited for 4 hours, just in case it was just slow but at the end I had to shut it down with power button.
Maybe will not make a difference but I'll give it a try once more. At least, I want to see if the "freezing" happens at the same point.
Just to mention that both the keyboard and the mouse did not respond after a couple of minutes of operation. I was just able to initiate the testing.

 

[tassos]

Ok. I've made the big step to remove the bluetooth card. The LCD connection was a tricky part but all good!

As I am writing now from my iMac after 10 minutes of use I can probably confirm that problem dissapeared!
So, I can assume that the cause of the problem was indeed that small and innocent, faulty, bluetooth card as TwoH suggested from the start!!

How a bluetooth, non working bluetooth card can destroy a sophisticated system as the iMac and not be able even to run the system in diagnostics mode is really frustrating but a different discussion!

In any case I would like to thank both TwoH and Benji888 for their suggestions and I hope that my thread will help others to identify and solve the same problem as I did.

Regards to the community!