iMac will not power on unless clock battery removed/reinstalled

Hello all,

I shut my iMac off one day. Next day when I restarted, I got a black screen with white text. Not knowing what to do, I pushed the power button and shut the computer down. I then tried to restart it, and that is when the problem started.

After much online research and opening up the computer, here are the facts. If I remove the little clock battery and then reinstall it, the computer will restart. Diagnostic lights 501. 502, 503 are all on when the computer is running. (Diagnostic light 504 is off because the monitor is not connected). When I shut the computer off, all the diagnostic lights go out. I understand that light 501 should always remain on if the computer is connected to a power source. I can only get light 501 to come back on by removing and reinstalling the little coin clock battery.

I removed and inspected the power supply, found no bad capacitors or burn marks, reinstalled power supply. I bought and installed a new coin battery for the clock. Still have same condition.

I appreciate any help that is offered. Thank You