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Model A1312 / Mid 2011 / 2.7 & 3.1 GHz Core i5 or 3.4 GHz Core i7 Processor, ID iMac12,2

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Why iMac 2011 mid 27' only run smoothly with an external monitor?

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Dear iMac superman

my second hand imac has been frequently reboots since June.

i found that it stops reboot, stop auto restart only when i connect with an externally powered screen. the youtube can run smoothly for 1-2 hours. But when i type something on search bar in google, there is a lag.

my question is:

  1. is the failure cause on power supply unit, graphic processing unit, logic board, or a mix of them?
  2. i run a nova benchmark test, gpu is zero score. does it mean GPU died? but my youtube runs smoothly,

please help. thanks.

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I would reconnect the internal display and then run the onboard diagnostics to see what it tells you.

Restart your system and press the D key to enter

Reference: Mac startup key combinations

I'm suspecting your discreet GPU has failed.

Diagnostics runs on the firmware driver for the Intel Intergraded graphics engine it has.

Your external display is able to run using the same Integrated graphics engine, but this time using the OS driver, as it we default to that.

You can also prove this using this great app gfxCardStatus

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Remove the unit power supply and check for faulty capacitors. It would not be surprising that a 11 years old PS needs a recapping. I just worked on this iMac model and even if the power supply was working there was 3 damaged capacitors. Top of the cap is doomed, its a sign the cap is failing soon or later.

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pmp calvin will be eternally grateful.
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