iMac 27" mid 2011 - how to determine hardware error

Hi

I have a mid 2011 iMac 27". A wonderful piece of hardware that has been this family's main workstation for 7 years.
but no longer.

I will no longer boot and seams to enter a reboot loop.
Fixing it would be very nice as I believe it may have a few more years of service for this family.

As far as I can google it is most likely to be one of the following errors:
1. Graphics card
2. Power supply

Is there any way to tell which defect I have?

Any help will be much appreciated.

Kind Regards J;-)

Could also be a hard drive failing. Do you have an external drive from which you can boot to see if that works? If it boots from an external drive, the issue isn't the card or power supply.

Things to try;

Safe Mode - https://support.apple.com/en-us/HT201262
Recovery - https://support.apple.com/en-us/HT201314, if your OS is 10.7 or later
Apple Hardware test - https://support.apple.com/en-us/HT201257
Reset NVRAM - https://support.apple.com/en-us/HT204063
Reset SMC - https://support.apple.com/en-us/HT201295

A lot will depend on which version of Mac OS X/OS X/macOS is running on your iMac, as that model originally shipped with Snow Leopard discs, but may have had Lion installed shortly after build date?

Exactly what Jake mentioned. Boot the computer from an external HD. This will help you figure things out.

And just in case the main partition of the HD is messed up only...try booting the computer into the Recovery Partition...by pressing the 2- key combo...Command + r when booting the computer.

- Nick

ferrarr said:

Things to try;

Safe Mode - https://support.apple.com/en-us/HT201262
Recovery - https://support.apple.com/en-us/HT201314, if your OS is 10.7 or later
Apple Hardware test - https://support.apple.com/en-us/HT201257
Reset NVRAM - https://support.apple.com/en-us/HT204063
Reset SMC - https://support.apple.com/en-us/HT201295

A lot will depend on which version of Mac OS X/OS X/macOS is running on your iMac, as that model originally shipped with Snow Leopard discs, but may have had Lion installed shortly after build date?

Click to expand...

El capitain here guys!!!