G4 400 dead - blinking DS5 LED - fans twitching...

Here's what the Service Manual says about the LEDs.

Logic Board LEDs: Power Macintosh G4 (PCI Graphics)

There are seven diagnostic LEDs on the Power Mac G4 (PCI Graphics) logic board. A graphic follows showing their location.

DS1 = ATA drive activity

If a CD and/or Zip are attached to the ATA bus, this LED will illuminate at power/on restart time for a few seconds. After that, it is unlikely it will illuminate again in a trouble-free system. If this light stays on for a prolonged period, this is a sign of trouble on the ATA bus. If devices are connected to the ATA bus and this LED is dark at boot time, that is also likely a sign of trouble. If a hard disk drive is attached to this bus, the LED flickers regularly indicating drive activity (reads/writes).

DS4 and DS5 = USB power for downstream devices

DS5 = USB port 1- lower
DS4 = USB port 2- upper
(There is no DS2 or DS3 on the board.) When illuminated, these LEDs indicate that the computer is providing power (+5 V) to downstream USB ports. USB devices do not need to be connected in order for these LEDs to illuminate. However, if one or both of the LEDs is not illuminated and a USB device is/was attached (and the system has completed its boot process), then the logic board has removed downstream power because of a trouble condition. Since the downstream power
is controlled individually for each port, it is possible to have these LEDs in a different state: one off and one on. This way if one port has troubled USB devices, the other port can still be functional. These LEDs are not immediately illuminated at the beginning of a restart or cold power-on.
During a successful/normal boot, the sequence will look like:
1. Unit is powered on and other LEDs will illuminate while DS4 and DS5 are off.
2. Then DS4 and DS5 on for a second or two
3. Then off very briefly (1 second)
4. Then back on (confirms secondary PCI bus is initialized and power now available) These two LEDs are often useful when troubleshooting an unpredictable system. If they stay on (step 4) then the secondary PCI bus has been initialized which happens relatively late in the boot/hardware initializing process.

DS6 = Ultra ATA bus activity

DS6 indicates Ultra ATA bus activity.

DS7 & DS8 = Power Indication

In a working unit, these LEDs should be in sync with each other: both on or both off. If one LED is on and the other is off, there is a problem.

DS9 = CPU Bus Request

When illuminated, this LED indicates that the CPU is requesting the bus. This LED will rarely glow as brightly as the others (like the Ultra ATA LED) given that the CPU is not always requesting the bus, and when it does, the period of time is short. As a result, you may need to watch this LED much more carefully than the others to tell if there is activity. If this LED is ever on continuously and bright, then the system is troubled and likely hung.

You might also check your internal memory battery & try a CUDA reset. See Mac PRAM, NVRAM, CUDA/PMU & Battery Tutorial

Cheers, Tom