Joshua, start with what Apple suggest:"
How to Reset the System Management Controller (SMC)
The System Management Controller (SMC) is a chip on the logic board that controls all power functions for your computer. If your computer is experiencing any power issue, resetting the SMC may resolve it. The SMC controls several functions, including:
Telling the computer when to turn on, turn off, sleep, wake, idle, and so forth.•
Handling system resets from various commands. Controlling the fans.•
Note that resetting the SMC does not reset the PRAM. Resetting the SMC will not resolve issues in which your computer is unresponsive—in these situations, restarting your computer will generally suffice. If your computer isn’t responding, perform these steps one at a time, in this order, until the issue has been resolved:
Force Quit (Option-Command-Escape).1.
Restart (Control-Command-Power).2.
Force Shut Down (press the power button for 10 seconds).3.
Resetting the SMC can resolve some computer issues such as not starting up, not displaying video, sleep issues, fan noise issues, and so forth. If your computer still exhibits these types of issues after you’ve restarted the computer, try resetting the SMC.
To reset the SMC on an iMac
From the Apple menu, choose Shut Down (or if the computer is not responding, hold the 1. power button for approximately ten seconds until it powers off).
Unplug all cables from the computer, including the power cord.2.
Wait at least 15 seconds. The SMC reset occurs automatically once the iMac has been 3. unplugged from the AC power source for several seconds.
Plug the power cord back in, making sure the power button is not being pressed at the time. 4.
Reconnect your keyboard and mouse to the computer.5.
Press the power button on the back to start up your computer.6.
Note: The iMac does not have a button on the main logic board for resetting the SMC, which eliminates the need to remove the computer’s cover to reset it (as required for some other models).
The iMac has four built-in diagnostic LEDs on the main logic board that can help you to troubleshoot the computer. The four LEDs are located to the right of the memory slot, under the hard drive data cable.
LED #1
Indicates that the trickle voltage from the power supply has been detected by the main • logic board. This LED will remain ON whenever the iMac is connected to a working AC power source. The LED will remain on even when the computer has been shut down or put to sleep. The LED will turn off only if the AC power source is disconnected or the power supply is faulty.
LED #2
Indicates that the main logic board has detected proper power from the power supply when • the computer is turned on. This LED will be ON when the computer is turned on and the power supply is working correctly.
LED #3
Indicates that the computer and the video card are communicating. This LED will be ON • when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement.
LED #4
Indicates that the computer and the LCD display panel are communicating. This LED will be • ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement.
No Power
The computer will not turn on. The display remains black and there are no sounds from the fans or drives.
1. Verify the power outlet is good. Plug a different device into the socket to ensure there is power, or plug the iMac into another outlet. Does the iMac power on now?
Yes: Resolved. Bad outlet.
No: Go on to the next step.
2. Check the power cord. Use a known good power cord. Does the iMac power on now?
Yes: Your power cord has failed. Replace the AC power cord.
No: Go on to the next step.
3. Check the connection of the power cord on both ends. Verify that the power cord is securely plugged into both the AC outlet and back of the computer. Does the iMac power on now?
Yes: You may have a loose fit to your power cord. Replace the AC power cord and test.
No: Go on to the next step.
4. Follow instructions in the General Information chapter to reset the SMC. Does the iMac power on now?
Yes: Issue resolved.
No: Go on to the next step.
5. Remove and reinstall the SO-DIMM memory modules, located in the bottom of the computer. Does the iMac power on now?
Yes: Issue resolved.
No: Go on to the next step.
6. Remove the memory access door and front bezel to gain access to the four diagnostic LEDs. See “Diagnostic LEDs” in the General Information chapter for complete instructions on this procedure.
7. Plug the power cord into the iMac and the power cord into the AC outlet. Check to see if LED #1 is On or Off.
LED #1 is On: This indicates that the power supply is getting good power from the AC outlet. Go on to the next step.
LED #1 is Off: This indicates that the computer is not detecting AC power. If both the AC outlet and the power cord are good, replace the AC/DC power supply. If the issue persists, replace the AC line filter.
8. Press the power button. Check to see if LED # 2 comes On, comes on momentarily, or stays Off.
LED # 2 is On: The Power Supply is functioning. Go on to the next step.
LED # 2 comes on momentarily or stays Off: Replace the Power Supply.
9. At this point in the Power On process, you should hear a boot chime and see that LED #1 and #2 are On. Do you hear a boot chime?
Yes: The power systems of the computer are working correctly. See “Troubleshooting No Video” in this chapter.
No: The logic board is not passing the Power-On Self Test (POST). Replace the SDRAM with known-good memory and test. If the issue persists with known-good SDRAM, replace the logic board." Hope this helps, good luck.
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