Dell optiplex 790 2-4 error no start up

You need to post more info about the problem. Include your operating system and anything you changed or added before this happened. Here's the lights chart- https://www.dell.com/support/article/en-us/sln284978/a-reference-guide-to-the-dell-optiplex-diagnostic-indicators?lang=en

The indicator is 2 and 4 and running windows 10 nothing has changed I've been editing videos with filmora and playing minecraft with nothing on it.

2,4 with blue power light would mean bad graphics card or error if you had one.  There's no 2,4 error code listed with amber light.

Try replacing the size 2032 battery if it hasn't been done in the last 5 years.  Low battery will cause all sorts of bizarre problems.  Duracell or Energizer is suggested as they last around 5 years.  Sony only lasted me 2.

If that doesn't work, then try the BIST test.  It's a way of testing internal system components.  https://m.youtube.com/watch?v=udGOe7LQJEI  

Okay, I did the test and every thing is perfect, I'll buy a new battery tomorrow, I lost one the one in my pc the other day.

By any chance is that good news - the error is gone?

Be sure to get that loose battery out of you PC before it shorts anything.

I should have been more clear it's the test on the power Supply that said every thing is ok. I found a working 20 yo graphics card but I dont have the cord for it but once in the error goes away so that's somewhat good news.

At least the BIST test narrows it down some.

Get a new battery in that thing if possible.

Power on with no RAM in it.  Does it still do the 2,4 error?  (Hopefully it shows a different error.)  If not reinsert the RAM, one chip at a time, booting up in between, until the error repeats.  Try a different chip if the error repeats right away.  Be sure to remember to power off and unplug anytime working on the inside of a PC.

After that, if error still persists, try reseating the CPU.

Beyond that could mean bad MB, or it needs to go to a PC shop, or both.

I take it went back to 2,4 with RAM reinserted?  Could possibly be a RAM problem.

Huh, did you understand what I meant with the RAM though?  Once you removed the RAM, the error code changed.  Once reinserted, 2,4 is back.  Though it should've gave a different error code, might be a RAM problem.  Going with "might," but not "likely" with the error code being 2,4.

Same thing but I do need better thermal paste!!!

I did the ram thing and it beeped and the 1 light was on, I will reseat the cpu and see what happens.

Could it just be the integrated graphics since I put a old graphics card in and the error was gone it booted but I can't use it I have no cords for it 

I will do more experiments with the ram to see if it will change 

Interesting, as integrated graphics are part of the CPU.