Powers up, but shuts down after 5 seconds

Here’s what happens: I hit the power button, and it fires up right away with the fan at full blast, but nothing ever comes on the screen and none of the LED’s on the front light up. After about 5-7 seconds of fan noise only, it shuts down. Here’s what I’ve tried:

• Hold power button down for 30 seconds, with and without the battery in.
• Start it without the battery in, charger only
• Reseat the RAM
• Stripped it all down and replaced (resoldered) in a replacement CMOS battery, exact replacement part. The old one seemed to be dead.
• Tried to run it on an external monitor
• Tested power cable, it’s outputting voltage as it should.

So all of the above fixes were tried, and the exact same thing happens, no matter what I do. With the charger plugged in, I’d expect the LED status lights on the front to light up at some point, or while it’s booting, but nothing. There is no reset button on this model.

Any ideas?