Resetting the PMU (Power Management Unit) on the logic board can resolve many system problems. Whenever you have a unit that fails to power up, you should follow this procedure before replacing any modules.
1. Disconnect the power cord.
2 Remove the user access panel from the bottom of the computer.
3 Press the PMU reset switch (shown below) once on the bottom side of the logic board and then proceed to step 3. Do NOT press the PMU reset switch a second time because it could crash the PMU chip.
4 WAIT ten seconds before connecting the power cord and powering the computer on. If the computer powers on, go to the next step. If the computer does not power on, there is something else wrong with the computer, refer to the symptom/cure, "No Power" in this chapter.
5. Run MacTest Pro and return the computer to the customer.
Note: This entire procedure resets the computer's PRAM. Be sure to reset the computer's time, date and other system parameter settings before returning the computer to the customer.
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