iMac won't turn on

@greezy91 start off by checking what Apple suggest:

No Power, Dead Unit

Symptoms Quick Check

No Power, Dead Unit

• No power

• No fan spin

• No startup chime

• No image on external display

• No hard drive or optical drive activity

• Caps Lock LED on wired keyboard doesn’t light when pressed.

1. Verify power source.

2. Verify power cable.

3. Listen closely for signs of activity from system including: rotating fans, hard drive or optical drive

activity, startup chime, etc.

Deep Dive

1. Power ON system. Verify if there is any indication that the system has powered up (fan rotation, hard drive or optical drive noise, backlight turns on).

Yes The symptom is powering up.

Go to Won’t Start Up.

No Go to step 2.

2. Disconnect AC cable Remove glass and LCD panel and locate the diagnostic LEDs in the middle of logic board. Reconnect AC cable to system. Verify if diagnostic LED #1 turns ON. You may alternately check for presence of a 12V DC signal between pin 4 and pin 1 of the logic board.

Yes Standby power voltage being supplied by power supply.

Go to step 3.

No Go to step 4.

3. Press power button while monitoring diagnostic LEDs. Verify if LED #2 turns ON and remains ON after releasing the power button. You may alternately check for a power request signal between pin 12 and pin 1 of logic board.

Yes Power supply functioning and logic board detects supply voltages and is powered on.

Go to Won’t Start Up.

No Go to step 5.

4. Disconnect AC cable. Carefully inspect the connectors between AC inlet and power supply, then DC

connectors and cables between power supply and logic board, for damage or poor connections.

Yes Reseat misconnected cable(s), or replace damaged one(s).

No Replace DC power cable harness. Go to step 8.

5. Inspect power button and verify if it is properly connected to the logic board.

Yes Go to step 6.

No Reconnect power button.

6. Disconnect power button from logic board. Inspect cable and connector for damage. Using a multimeter (set as ohmmeter), verify continuity between the two pins of the power button when it is pressed.

Yes Power button is functioning correctly. Reconnect power button to logic board and go to step 7.

No Power button faulty. Replace rear housing (which includes power button).

7. Reconnect AC cable Press power button and Verify if system turns ON. You may alternately check for presence of a 12V DC signal between pin 11 and pin 1 of logic board when power button is pressed.

Yes Power supply functioning. If system still won’t boot, go to Won’t Start Up.

No Replace DC power cable harness. Go to step 8.

8. Press power button and verify if system turns ON.

Yes Issue solved with replaced DC power cable harness.

No Replace power supply.

Go to step 9.

9. Press power button and verify if system turns ON.

Yes Issue solved with replaced power supply.

No Reinstall power supply and replace logic board.

If No Power symptom persists after all steps have been followed use minimum configuration troubleshooting to proceed. Try disconnecting hard drive(s), optical drive, AirPort, , SD card modules and Bluetooth/Camera cable to determine if one of them is preventing the power supply from functioning.