2 of those errors are related to the CPU, either not found or a CPU error. The other error is a memory error. With those number of errors it could very well be a motherboard. You can try reseating the RAM modules and removing and reseating the CPU, just be sure to clean any thermal paste and reapply new paste. I cannot see the CMOS battery being the problem but just to be sure remove the battery (coin cell), leave it out of the system for 15 minutes, then install a new battery.

As far as replacement parts go. This system is nearing 8 years old and putting that kind of money into parts may not be the the best course of action. Start thinking about replacing the system with a new, faster and more powerful system.

https://www.dell.com/support/article/us/en/19/sln284978/a-reference-guide-to-the-dell-optiplex-diagnostic-indicators?lang=en#2009_to_2012

https://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_optiplex_desktop/optiplex-790_owner%27s%20manual_en-us.pdf

JOcean, thank you for your insights.

If I can fix it for $20 - $30, I'll keep it because the 790 and I have a lot in common. I'm old and slow too.

As you said, the thing is eight years old and I fully realize that in "computer years" the box is ancient. But even though it's only a couple of steps ahead of the abacus, it's really all I need.

I have a decent Samsung Galaxy Tab S which is getting old too. But right now it's covering me for email, searches, information gathering and the like. I'm on it right now as a matter of fact.

Later today I'm taking the 790 to a friend who will probably be able to figure out what is wrong with it. If he comes up with a definitive diagnosis I'll post what he said here. I know there are other 790 owners here who may find the information interesting or even useful at some point.

And thanks again.

Its time to "let go".. This coming from someone that's probably older than you (I'm 81).

I fully understand the attachment you have to an older PC, but if a) there is any question where the problem lies (i.e. it could be multiple issues = multiple $$$ purchases) or if the repair runs anywhere north of $50, then you really should take a look at a newer system.

That doesn't have a to mean a lot of money, as just recently Dell Refurb (which sells good stuff - I buy there all the time) had a President's Day 40% off sale that dropped some quad-core 4th-generation i'5's (very fast for the money) down to $100-$150 depending on the config.

Actually, I just looked and it's 40% off their Optiplex 7020 line, not the entire desktop line, but these are the 4th-gen Intel CPUs that are such a good value right now - looks like all these sales have taken most of the good stuff though.

For a limited time: 40% off OptiPlex 7020 Desktops (excl. Clearance), free ground ship. Use coupon code: DT7020SALE

https://www.dellrefurbished.com/desktop-computers?filter_model=1564

Systems like this are still supported and repaired etc.  HOWEVER it costs $75 an hour plus travel time to fix onsite.

When you can get another unit WORKING With windows 10 for less than the cost of windows license it makes no sense to repair it.

https://www.walmart.com/ip/Dell-Optiplex-790-Windows-10-Pro-Desktop-PC-Tower-Core-i5-3-1GHz-Processor-8GB-RAM-1TB-Hard-Drive-with-DVD-RW-Refurbished-Computer/374746488

Why buy another ancient 790 when you can get a far newer 7010/7020/9020 from Dell Refurb with exponentially better performance?

There have been repeated 40% off sales there and before there was a big rush on them, sub-$200 was standard for a desktop.

The main reason to get another 790 tower is that the hard drive can  be moved from the broken unit to the new one and everything works.

Once you go past 7010  9010 the power supplies become 8 pin proprietary.

This is not really an issue with a MODDIY adapter but for some its a deal breaker.

Under $199 for the whole system with windows 10 Pro is basically the cost of windows and the computer is free.

Seems like a big negative to stay locked in 2010-performance+features forever when you can just move your files, email and programs over to a new PC and jump forward.

It's like when I buy a new SSD - it's never going to be smaller or slower than the one it's replacing.

Running office applications and using the network doesn't get faster with newer faster hardware.

My  clients in almost all cases want it to work and do not want to spend the time reinstalling and re configuring a machine.  The basic reason for this is very simple.  Reinstall and reintegration is not free.  Sysprep images of hard drives is standard and if you have all the same unit in an office a standard image gets people up and running by swapping out hardware is faster and cheaper than onsite repairs.

A newer GPU can bring an older system to a better level of performance without a great increase in cost.  The 790/990 also does not have proprietary 8 pin power supply.  The newer 3040 3050 3060 7040 7050 7060 models further complicate this with 290W or less tiny proprietary power supplies.  9020 tower is at least modifiable to a standard power supply.

Newer Intel Graphics is better but even the GT1030 is killing the GT2 and GT3 intel graphics.

https://www.youtube.com/watch?v=o8BIWIPBt18

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@speedstep wrote:

Running office applications and using the network doesn't get faster with newer faster hardware.

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Come on, of course it does.

Excel and Access are performance hogs when using extremely large files (like the financial/bank/trading environment), and the CPU processing of a 6-core 8/9-series far outstrips anything else I own right now. And we're talking multiples faster at processing large Excel or DB files.

And yes, these all have 16GB DDR3 or DDR4 and an SSD, naturally. The only real difference is the older/slower CPU and chipset + subsystem.

On a fast 4-series i7, I used to sit there watching the screen refresh over and over and over, waiting for formulas to update, but now on an i7-8700 the formula updates just whip by in no time. Same with Access - queries and data loads are significantly faster, and I also program a bit and compiles are also noticeably faster on the newest CPUs. I also have an old 2-series i7 dual-core that will virtually lock up during these processes, showing "Excel is not responding" for maybe 5 minutes before it was done. That same job takes me maybe 30 seconds on the new system. To me, that's no joke.

In fact, office work number-crunching is the main reason to upgrade CPUs (within recent vintage), as games are almost entirely GPU-limited.