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Considering the age of the system, remove and replace the CMOS battery with a brand new one which can be purchased at almost any retailer where batteries are sold. And leave the battery out at least 15 minutes before installing the new battery.

Some clarification needed.

"Optiplex 3020 seems to power on but the motherboard displays a solid orange light"  Seems to power on - Does it boot?  Orange light on the MB - Definitely not referring to power button orange light?

I don't recall 3020 having an orange light on the MB.  If it does, that just means it has power.

Solid amber power light means possible bad MB.

Blinking amber power light can mean bad PSU.

With the PC off, if you push the PSU test button, does it give you the green light?

@Ryan.y 

power button diagnostic led-status

Solid amber. Power connector not installed properly or bad PSU.

Solid amber. A possible system board failure has occurred.

"Solid amber. Power connector not installed properly or bad PSU."  Blinking amber.

@speedstep 

Umm, you're link goes here:

Dell OptiPlex Diagnostic Indicators 

3020 was introduced around 2013-14.

@bradthetechnut 

One way you can localize when a system was made is what OS it comes with.  Systems before 2007 came with XP aka 2002 to 2006 was XP. WIN7 came out in 2009 so models released from 2007 will be vista and 2009 will be win7.   From 2009 to 2011 it stayed 7 and changed to WIN8 in 2012.  All models Released in 2015 had WIN10 build 1507. So models introduced in 2012 will have WIN 8.0 Genuine labels not WIN10 labels.

Diagnostic LED indicators are similar or the same for series like optiplex, precision, dimension, etc.  Windows vista is 2007, Windows 7 is 2009, Windows 8.0 is 2012.  3020 specifically is 2009 to 2014. I know this because there are no WIN10 genuine stickers on 3020 7020 9020 but there are WIN7 COA and Stickers on 3020 7020 9020.  If any were made/released in 2015 they would have come with windows 10  1507.  So I'm not sure what the point of bringing that up is.  The T1600 site has a table and explanation for Power Button LED and the Diagnostic LED's.  Visually the 4 diag LED's are the same for most dell models all the way back to 2002 and Windows XP .  3010 7010 9010 is 2011 with windows 7 and then 8.  Again Vista came out 2007, WIN7 was 2009, WIN8 2012, WIN10 2015.

1. Diagnostic LEDs for the OptiPlex series (2020 to Present)
2. Diagnostic LEDs for the OptiPlex series (2012 to 2020)
3. Diagnostic LEDs for the OptiPlex series (2009 to 2012)
4. Diagnostic LEDs for the OptiPlex series (2005 to 2009)
5. Diagnostic LEDs for the OptiPlex series (Prior to 2005)
6. OptiPlex Audible Beep Codes
7. Glossary of Acronyms
8. Additional Information

@bradthetechnut 

"3020 was introduced around 2013-14."  no that's incorrect.

Where the diag led's are on an optiplex changed from the rear to the front around 2006 but the 4 led meaning stayed the same well into 2012.  When a system was released from 2006 to 2014 has no bearing on what the LED's mean.  In general solid amber power button can be bad power supply or dead cpu / motherboard.

Optiplex 3020 Spec Sheet

Lists Ubuntu, WIN7, WIN8. No mention whatsoever of WIN10

If it was introduced in 2013 there would be models with WIN10 genuine labels.  There are not. its a 2012 model.

Since the 5 year system design life cycle has releases from 2012 to 2014 thats why there are no WIN10 versions or OS listed in the spec.

Thank you for the suggestion, I will buy a new cmos battery and test it out !

Yes the motherboard has a solid orange light displaying. I hope it is  just indicating power and no the power button is white when its on. Also no the psu does not light up green when the test button is pressed, is that a bad thing ?