Boot to gray screen Apple troubleshooting list followed down to replacement of PRAM Battery after multiple other solutions.
Very smooth after finding or buying missing tools from local hardware and iFixit.
iFixit directions (iMac Intel 20" EMC 2133 and 2210 PRAM Battery Replacement) are very accurate but two suggestions:
1) the front iMac glass requires suction or other upward force to remove it, but the glass itself is very light. Be careful not to pull it off and lose control of it.
2) After removing 12 Torq screws, I didn't know how big the front bezel would be. I thought it might be just the thin rim of aluminum where the screws were removed. In fact, it's the whole front aluminum case down past the Apple logo, which is why the access door to the RAM had to come off first. A prior commentor suggested something about removing the bezel/front case instead of disconnecting the microphone cord. This is what I did and it is a great idea to avoid damaging the cord trying to take it apart.