Mac Pro 4.1 won't turn on - 5V STBY only diagnostic light

@tbrep try these steps and let us know what you get:

1. Verify that connection between power supply and backplane board is undamaged and secure. Verify that power button and front panel board cable connections to the front panel board are undamaged and secure. See Functional Overview.

Yes

Go to step 2

No

Reseat all connectors and retest.

2. Connect AC power cord to the computer. Verify the 5V STBY LED illuminates when you press the DIAG button on the backplane board.

Yes

STBY LED illuminates, but computer won’t power ON.

Go to step 4.

No

Suspect power supply failure.

Replace power supply and go to step 3.

3. Verify the 5V STBY LED illuminates when you press the DIAG button on the backplane board.

Yes

Issue resolved.

No

Go to step 5.

4. Disconnect the front panel board. Momentarily jumper the pictured SYS_PWR solder pads on the backplane board. Verify the computer turns ON.

Yes

Front panel board or power button faiure. For front panel board, use code M01. To verify power button, go to Power Button Stuck.

No

Go to step 5.

5. Reduce computer to minimal configuration. Verify the 5V STBY LED illuminates when you press the DIAG button on the backplane board.

Yes

Suspect a module that has been removed. Reinstall modules until symptom reappears. Replace affected module. Then go to step 6.

No

Replace backplane board and retest.

6. Reconnect the processor board without processors. Press power button and verify computer turns ON. Note that you can identify the computer has started up by fans and LEDs; however, there won’t be any other activity since there are no processors.

Yes

Add processor A and retest.

If computer power remains stable, the power issue is resolved.

No

Go to step 1.