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Model A1311 / Mid 2011 / 2.5 & 2.7 GHz Core i5 or 2.8 GHz Core i7 Processor

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intermittent start issue - only HDD spin-up

The iMac was running for years, but since some months, it starts quite unreliably. After pressing the power button I can hear the HDD spins up, then stops and after a moment, spins up again. No video signal, no sound. It can do this for 30min and suddenly the boot process continues. Once the machine is started, it works flawlessly even over a long period. I found out that it's possible to influence and 'force' the start if I press, hold and release the power button while the iMac is in the trying-to-start loop - but I need some luck to gain the proper timing for that to happen.

Checking the SMART values shows nothing special beside an extrem high spin up / power cycle count.

Where should I start to search? Do the symptoms point to the PSU?

TIA!

Tobias

Update (06/11/2016)

Ok, I did some more tests with the iMac opened up:

- 1st Diagnostic LED is constantly on while device connected to power, no flicker

- I press the start button

- 2nd LED turns grren, fans and HDD start up for about 1 sec.

- 2nd LED, fans and HDD stop again for about 3-4 sec

- the machine continues altering between the last two states about 1-4 times, then the boot continues, you hear an sound from the SuperDrive, the start chime, the 3rd LED comes up and everything seems normal.

- if the iMac is hot (was running before) it seems the trouble is worse. It can take some minutes instead the 1-4 attempts till it finally starts

I tried booting with the hard drive removed as well as with it exchanged with a SSD, the error continues. Starting from either internal or external or AHT does work once the first boot phase is passed. I had a look to the power supply, there are no obviously damaged (blown up) capacitors on it.

What to test next? PSU? logic board? can such symptoms be caused by faulty RAM? AHT does report an HD thermal sensor problem with the SSD (have ordered the OWC sensor cable to fix that) but nothing else.

Answered! View the answer I have this problem too

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To whom it may be of interest: in the end it was a faulty RAM that caused the startup problems. Once started, even extensive RAM tests didn't show errors and the system was usable stable. But after replacing the RAM in question, all the intermittent startup issues went away. Thanks for your help, anyhow!

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I suspect a failing hard drive. To test, install a system on an external hard drive or USB flash drive. Hold down the Option key on start up and select that drive. If it boots well, that will confirm that it is a hard drive issue. If so, ask another question about replacing it as there are issues to address on replacing the drive in this model as the drive has pole priority heat sensor issues.

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3 Comments:

Dear Mayer,

thanks for your quick reply. Unfortunately, the boot doesn't get that far, that I can select the boot device. The boot process seems to abort in a quite early phase.

As I said, the System is running well once started and even an extended SMART test did not show any abnormalities. I may get some more hints once I open the iMac from the Diagnostic LEDs. Then I can also try to start w/o the HDD to test your assumption. Would be nice to be able to order the most likely necessary spare part before I open the machine, thus.

Best,

Tobias

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I understand, but you said you do get it up once in a while. At that time put a system on an external. The progress bar on the bot and when it stops can tell us things.

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This machine is also notorious for bad video cards. If it stops booting about mid way on the progress bar, that's when it's checking the GPU. I've had a lot of success with a hot air rework station reflowing the solder on the GPU. It did one last month and it worked great and is still running. Did another one and it worked fine for two weeks then started getting blue vertical lines on the screen and has stopped mid-boot. Will rework it again next week.

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Tobias Bussmann will be eternally grateful.
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