iMac refuses to start

http://docs.info.apple.com/article.html?artnum=106464 will give more information on startup problems.

Unfortunately, the symptoms suggest a failed PAV board, especially since the problem appeared out-of-the-blue.

If you're comfortable diving into the "innards" of the machine, a PAV board can be purchased on eBay and replaced by doing a search on the net for iMac PAV board replacement.

David

Hi and Welcome,

When I've seen that power button problem it's pretty obvious since the travel on the power button is clearly limited. Actually in a few instances there was chocolate sauce and the like restricting the travel.(I do work in a kindergarten) You must have the bottom off since you reset the PMU just check to see that there's a little pretravel in the inner button before you feel it contact the tactile switch.

I also notice that in certain versions of the Service Source the following has gone missing. It's referred to but a search turns up nothing. In other versions that I have with exactly the same title it's shows up:
"Resetting the Logic Board
Resetting the logic board can resolve many system problems (see
the Symptom/Cure tables for examples). Whenever you have a
unit that fails to power up, you should follow this procedure
before replacing any modules.
4 Unplug the computer.
5 Press (reset) the PMU button on the logic board.
6 Plug the power cord back into the computer and press the
power on button on the front of the computer. If the computer
powers on, go on to step 5.
Note:
If the computer won’t power up, reset the PMU again,
this time with the power cable plugged into the computer and
go on to step 4.
7 Press the power on button on the front of the computer again.
If the computer won’t start, go to the next step. If the
computer does start, you may have a bad battery.
8 Remove the battery from the logic board. Check that the
battery is good.
9 Wait at least 10 minutes before replacing the battery.
10 Make sure the battery is installed in the correct +/-
direction.
11 Power on the computer and test the unit.
Note:
This procedure resets the computer’s PRAM. Be sure to
check the computer’s time/date and other system parameter
settings afterwards."

If none of that helps David is probably right it's the PAV. My current fix for this however is to swap the board into a working 350 since there are a ton of the 350s becoming available for next to nothing as schools and graphics people phase out these iMacs. We however are still using them. Even running GarageBand and Tiger on them.

Richard