I've had a couple of G5 towers that when the power button was pressed, the white light would come on, and the fans would start, but no startup chime and no boot. No flashing of the white light - it would come on and stay on.

Based on this old posting at MacRumors, I tried the hair dryer trick, and the machine would then boot and run normally. I didn't even need to take the side of the case off the machine. I could blow the hair dryer through the front of the machine for 15-30 seconds and then all would be well.

This was only necessary if the machine was shut down and stayed off for a period of time. Quick restarts such as for system updates, etc. were not a problem.

I don't have these machines any more, but I can vouch there there is some kind of thermal issue there.

Having tried the heat gun trick on some white Macbooks for video problems with small success, I can see how this "repair" on a G5 MB would be problematic.