Powermac G5 won't power on, no white power led, fans spin however.

@lesander check this and see if that is what your system does:

Power Mac G5 (June 2004): Power-on LED illuminates when pressed in, but goes out when button is released, fans spin, and there is no boot tone or video

• Verify that the processors are properly seated.
• Verify that the processors’ mounting screws are properly tightened.
• Remove the processors and check for damage to the logic board under the processors. If you see damage, replace the logic board.
• Remove lower processor. If the computer starts up correctly, go to step 5. Otherwise, go to step 6.
• Replace lower processor.
• Replace upper processor.
• Check for trickle voltage on the power supply connector. Refer to “Power Supply Verification” in this chapter. If verification fails, replace power supply.
• Replace logic board.

Power-on LED illuminates when power button is pressed and fans spin continuously but there is no boot tone or video

1. Reseat video card. (Make sure video card is fully inserted in connector and end of card is secured by the connector latch.)
2. Verify speaker cable is fully seated.
3. Reset logic board. Refer to “Resetting the PMU on the Logic Board” in this chapter.
4. Reseat the processor
5. Replace the processor
6. Replace logic board

Resetting the PMU on the Logic Board

The PMU (Power Management Unit) is a microcontroller chip that controls all power functions for this computer. The PMU is a computer within a computer. Its function is to:

1. tell the computer to turn on, turn off, sleep, wake, idle, etc.
2. manage system resets from various commands.
3. maintain parameter RAM (PRAM).
4. manage the real-time clock.

Important: Be very careful when handling the logic board. The PMU is very sensitive and touching the circuitry on the logic board can cause the PMU to crash. If the PMU crashes and is not reset, the battery life goes from about five years to about two days.

Note: For the location of the PMU reset button, see “Logic Board Diagram” in the Views chapter.

Many system problems can be resolved by resetting the PMU chip. When you have a computer that fails to power up, follow this procedure before replacing any modules:

Disconnect the power cord and check the battery in the battery holder. The battery should read 3.3 to 3.7 volts. If the battery is bad, replace it, wait ten seconds, and then proceed to step 2. If the battery is good, go directly to step 2.

Press the PMU reset button once and then proceed to step 3. Do not press the PMU reset button a second time because it could crash the PMU chip.

Wait ten seconds before connecting the power cord and powering on the computer. If the computer does not power on, there is something else wrong with it; refer to the “Startup Failures” section of “Symptom Charts” in this chapter.

Note: The above procedure resets the computer’s PRAM. After resetting the PMU, be sure to reset the time, date, and other system parameter settings.

If you want to check the power supply, then this might come in handy:

Power Supply Verification

To power on, the computer’s logic board requires a “trickle” power of +5V. If the system fails to power on, first reset the PMU. Then follow the procedure outlined below to determine whether the problem is related to the power supply.

Note: To verify the power supply, you need a volt meter.

• Remove the power cord from the computer.
• Open the computer, lay it on its side with the access side facing up.
• Remove the front inlet fan assembly.
• Remove the black plastic cap covering the 24-pin power supply cable connector (the P1 or largest connector). The cap fits tightly over the connector. To remove it, rock the cap gently forward and backward as you lift up.

Note: This is a top view of the connector when it is plugged into the logic board.

• Press the release latch on the connector and disconnect the cable from the logic board.

• Plug a known good power cord into the computer. Do not turn on the computer.
• On the power supply cable connector: connect the black lead of the volt meter to pin 23 and connect the red lead of the volt meter to pin 1.

Note: This is a bottom view of the connector when it is unplugged and facing up.

The volt meter should measure approximately +5V. If you do not get a reading of +5V, recheck the volt meter connections and measure the voltage again. If the voltage is still not present, replace the power supply.

Pinouts

Power Supply P1 Connector

For now you do not want to replace anything but just check to see if you can narrow it down.