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Unplugged iMac, removed 2g memory modules, replaced with 4g, no response from power button - reconfigured to old memory modules and still no power on -
Tried resetting PMU several times - attached external USB hard drive and no power light on drive so no power is coming through the USB port.
Any idea what to do next ??
iMac 21.5″, macOS 10.12
Make sure the power cord is firmly connected to the back of the iMac. Also double check the power cord is plugged into a working electrical outlet. If you are using a surge strip make sure the surge strip is turned on. Try using another electrical outlet. Try using another AC cord.
If these things don't work, then remove all the memory and try powering on the Mac without any memory installed. If the iMac powers on you should get three beeps for no memory. If you don't get three beeps, then either you have a power issue as described above or you have some sort of hardware issue with the iMac such as bad power supply or bad Logic Board (maybe the memory you used is bad and damaged the Logic Board).
Make sure the power cord is firmly connected to the back of the iMac. Also double check the power cord is plugged into a working electrical outlet. If you are using a surge strip make sure the surge strip is turned on. Try using another electrical outlet. Try using another AC cord.
If these things don't work, then remove all the memory and try powering on the Mac without any memory installed. If the iMac powers on you should get three beeps for no memory. If you don't get three beeps, then either you have a power issue as described above or you have some sort of hardware issue with the iMac such as bad power supply or bad Logic Board (maybe the memory you used is bad and damaged the Logic Board).
mid-2011 20.5" iMac has no power following RAM upgrade
