iMac 27" Late 2009 freezes during start up

[JorWesFlow]

iMac 11,1 2.66GHz i5

This started happening a few months ago and became progressively more frequent. It got the point where it now only boots into safe mode.

So I ran Apple Hardware Test and it would fail and give error 4MOT/4/40000003: HDD-1342 (the last four digit number would vary when the test was run again).
I Googled that code and found some different approaches to trying to solve it. One person said the HD fan was loaded with dust and cleaning that could fix it. So I opened up the iMac and blew out two cans of compressed air—yes, there was a lot of dust in there. Now Hardware Test now passes without error, yet the iMac will still freeze during start up as it had before.

So I tried swapping in a different HDD with a fresh install of Mac OS 10.11.4 (most current as of now). But the same issue happened. It would freeze with the boot progress bar a little past halfway, yet you could hear the male assistant voice speaking as it should upon first boot of a fresh OS install.

I then tried an SSD and same issue.

Some think it's the HD fan, but it works fine, and even better now that it's clean I assume.

So maybe that "4MOT..." error code was an unrelated issue to what's causing the freeze? Maybe that's a fair assumption to make since it's passing the Hardware test now. Any ideas on what the freeze is all about?

 

[stuartak]

iMac 11,1 2.66GHz i5

This started happening a few months ago and became progressively more frequent. It got the point where it now only boots into safe mode.

So I ran Apple Hardware Test and it would fail and give error 4MOT/4/40000003: HDD-1342 (the last four digit number would vary when the test was run again).
I Googled that code and found some different approaches to trying to solve it. One person said the HD fan was loaded with dust and cleaning that could fix it. So I opened up the iMac and blew out two cans of compressed air—yes, there was a lot of dust in there. Now Hardware Test now passes without error, yet the iMac will still freeze during start up as it had before.

So I tried swapping in a different HDD with a fresh install of Mac OS 10.11.4 (most current as of now). But the same issue happened. It would freeze with the boot progress bar a little past halfway, yet you could hear the male assistant voice speaking as it should upon first boot of a fresh OS install.

I then tried an SSD and same issue.

Some think it's the HD fan, but it works fine, and even better now that it's clean I assume.

So maybe that "4MOT..." error code was an unrelated issue to what's causing the freeze? Maybe that's a fair assumption to make since it's passing the Hardware test now. Any ideas on what the freeze is all about?

Does it freeze on the apple logo , or on a clear "white" screen? Unfortunately freezing up on boot can be an early indication of GPU card failure. Try connecting an external monitor via the mini display port and see if you get a different video display.

 

[JorWesFlow]

Does it freeze on the apple logo , or on a clear "white" screen? Unfortunately freezing up on boot can be an early indication of GPU card failure. Try connecting an external monitor via the mini display port and see if you get a different video display.

It freezes on the Apple logo in about the middle of the booting progress bar. Same spot every time.
GPU failure: could that explain why safe mode works? Because it's booting without the GPU and just using the integrated graphics?

 

[Significant1]

Safe mode does not load the graphics driver and therefore does not activate hardware acceleration from the gpu. Instead the cpu handle the screen drawing in software. I have changed graphics adapter in my 2009 iMac, it was still able to boot in safe mode. It had also visual artifacts on the screen, when it froze though.

 

[JorWesFlow]

It had also visual artifacts on the screen, when it froze though.

Yeah I have not seen any artifacts or really any other typical signs of a failing GPU. My late 2011 MacBook Pro had a failed GPU (fixed through Apple repair program) so I am definitely familiar with those common signs of a dying GPU...