Yesterday I bought and installed a 120mm (main system fan) and an 80mm (power supply) to replace the loud stock fans. After this it won't boot at all. When I press the front power button it just powers up for 1-2 seconds and then shuts right down again.

I tried taking out the mobo battery then pressing the pmu reset button just once and leaving it overnight. Then this morning I put the battery back in plugged power back in and tried to power it up. Same problem..

I have also tried unplugging the drives and new fans but still no go.

Any ideas or tips?

For the record its a G4 400 with a G4 1GHz upgrade, 1GB RAM, 2x80GB HD, and a DVD-RW

I'd reseat everything (processor, RAM, et cetera), make sure it's all down tight, do the resets, and then see if it works. Also try plugging in the old fans. Weirder stuff has been known to happen.

[censored]! Anyway.. everything is OK now

Macs can really be little bitches at times.

As with all truly exceptional things. Capote comes to mind.

sounds like the g4 is looking for the psu fan certain older computers are rigged to there fans like dell computers for example if you have a case fan go out and try to replace it without a dell certified fan the computer will start and say on the screen the system fan is damage or not responding system will shut down in 2 sec..

same thing i guess apply's to your mac as well