Chosen Solution
I have a lovely but elderly iMac and it doesn’t get to the chime when you turn it on. With the screen dismantled from the computer the fans turn fully on as expected, and connecting the screen temp sensor quietens them. Voltages on the pins for the PSU appear odd - pin2 and 9 is 3.5V to 3.8V. Pins4, 10, 11, 12, 13 are at 12V and pin 6 is at about 13mV. Non of the logic board status LEDs is illuminated. I can see them but there is no signal from any of them… any thoughts?
In conclusion, it is most likely that your iMac’s motherboard has failed. The voltage readouts on the motherboard are normal, which indicates that your iMac is not turning on because of a component failure, or the motherboard failing. You may want to try to reseat components on the motherboard such as the processor, memory, and GPU. Otherwise, you will most likely need to replace your motherboard. Here is a motherboard replacement that you can buy online: https://www.ebay.com/itm/164923592908?ep… Here is an iFixit guide on how to replace your motherboard: iMac Intel 21.5" EMC 2308 Logic Board Replacement
