There is not a lot of documentation on the CUDA or EGRET chip in terms of troubleshooting. How does it work? Well, I’ll do my best to document that here.
One of the core uses of the CUDA chip is soft power for AIO Macs like the 550 / 575. The chip is provided with a phantom power on pin 13. There is no other voltage provided to the board at this time.
If you have a Macintosh that won’t power on, you can try powering it on through the 5v rail. If you see it drawing an amp or so of power, you know the board is not dead.
It is not uncommon for the CUDA chip to be victim to bad capacitors. This usually leads to the board not booting. It is always worth reflowing the CUDA chip before doing anything else. Ideally you want to remove the chip and clean the pads if they are corroded, but a quick reflow might be all it needs.
The CUDA is a Motorola MPU which is powered by an external oscillator. I have seen these go complete bad. I’ve also seen where the connections are bad. You want to check the connections to the oscillator as if they are bad, the MPU cannot run.