Is my iMac's GPU going bad?

[e93to]

I have a late-2012 21.5 inch iMac. I notice there is a sudden and short flash of several black horizontal lines on white screen when I'm turning off the computer. The computer works fine though when it's on. I don't notice anything unusual. However, I have to mention that it takes too much time (maybe 30+ seconds?) to actually shut down the computer.

Is GPU going bad?

Thanks in advance.

 

[Intell]

That is likely normal when it only happens when powering off the machine.

 

[e93to]

Hello. Thanks for quick reply. I hope it's normal, but my iMac didn't do such thing until a month ago. Normally, it was just plain white screen smoothly dissolving away without any flashing horizontal black lines. But now, there are black lines flashing against white screen whenever I shut off the iMac. Also, I noticed it takes too much time to shut off the iMac. As a matter of fact, my mid-2009 MacBook Pro with 10.6.8 turns ON faster than my iMac can shut off...

However... the computer itself is stable and fast when it's on... Strange.

 

[Intell]

Did you recently install any updates or change the OS version on your iMac?

 

[e93to]

My iMac is still on Mountain Lion, which came with the computer at the time of purchase. And I installed the OS update a month ago, which was released along with 10.10.2 update. I also installed Yosemite on partition 1.5 months ago or so. And, strangely, that Yosemite partition become core volume (as documented on this thread: https://forums.macrumors.com/threads/1741742/)

Other than that, I didn't do much to my iMac other than installing/updating some apps. Newly installed apps within the past 2 months include Bitdefender, Batman Arkham City and Fantastical. They were all downloaded from App Store.

 

[Intell]

Did this start happening after you installed 10.10.2? If it did, it's possible that the EFI update included with that is making it does what it does now.

 

[e93to]

Sorry for the confusion. By "OS update," I meant Mountain Lion Security Update 2015-001, which was released at the end of January.

I still did not update Yosemite partition to 10.10.2. I haven't booted into Yosemite for awhile now.

 

[e93to]

Called Apple, and I was told that logic board has to be replaced.....

 

[Bomb Bloke]

Bet if you ran the symptoms by different Apple reps (without giving them a case number / serial so they can parrot the last response at you), you'd find more than a few of them agree with Intell - as I do. This is common behaviour for a lot of systems.

 

[MCSN]

a couple of lines does not a logic board replacement make. they can be pretty significant lines for it to be logic board i understand, I've only seen one and that was my mbpro. its been replaced. with partitioning and start up or shut down it could be like bloke says.

did they see the unit, or just hear your description?

 

[e93to]

I just gave the same description I provided in this thread - "several lines flashing on white screen once before shutting down." I did not bring my iMac to an Apple for diagnosis yet.

So far I have not seen any abnormal "lines" when iMac is on and being used. Maybe these "flashing lines" before shutting down are normal... I'll just wait, and I'll bring my iMac to Apple store if I start to see random lines when iMac is being used.