optiplex 9020 will not power up

steady green on BIST does not mean psu is definitely good.  First try replace CMOs battery with a new one and reset CMOs jumper on motherboard to clear CMOs settings.

How to run self-test using the button on the PSU

1. Turn off the system.
2. Press the BIST button on the system and check whether the LED lights up.
3. The light should be solid on not flickering or flashing and the power supply fan should also turn on to speed. If the fan is not spinning up properly this should be considered a BIST failure even if the LED lights up.

• If the BIST LED lights up, then the power supply can deliver power to the system but something else is impeding the POST process. The next step is to check whether the power button might be at fault. If it is, not you should go to the following article for No POST troubleshooting.

With the unit plugged in, pushed the BIST button - nothing happened.

So does this imply the power supply is bad?

If the BIST LED does not light up, then the power supply can't deliver power to the system and could be caused by any component connected to the power supply or the power supply itself.  Chances are bad psu.

What color is your power light blinking, amber?

Dell BIST Test YouTube 

AMBER

Hi @Forgottenname,

In the 1st post, you said the PSU green light is on.  In your 2nd post, you said nothing happened when test button was pushed.

Blinking amber (power button) is a sign of a bad PSU.  However, the BIST is there to make sure it's not another component causing the problem.

Thanks for help

Summary. Led light on rear steady green, cooling fan comes on.

Re power light in front. Blinks amber. Light blinks twice, pause blinks 3 times, pause and repeats.

Replaced power supply unit., same problem. 

Thanks 

MB is more likely to go before the CPU does.

I have the exact same issue as Forgottenname. Blinking amber power switch. Do you have a solution for me?

Dell Micro 920. Plugged in another external power supply and still no light on power button and fan does not go on and nothing else showing signs of power.

Thanks

Bob

You mean "9020"?  If it's not working with at all with another Dell OEM PSU, MB may be dead.  Impossible to diagnose and pinpoint exact cause from here.