Apple Diagnostics and failing power supply

Yes. Power supplies have many components, and especially for laptops and Mac Studios, and iMacs with the proprietary connectors. Fraying on the cable can be a problem, and with magsafe connectors, you also have a magnetic shield on the port that cane get loose with age or in the process of taking apart. The only part the diagnostics can realistically detect is the circuit board part that controls where the power goes on the logicboard. Water and moisture from liquids can make mincemeat out of any power supply. Some computers have a natural tendency to condensate inside, which can yield to the demise of power supplies. With age, capacitors can break down too, and lead to issues with inverter connections on the display.

If an error is detected, it is to be believed. As a computer repair person, I don't just look at the diagnostics but any symptoms, are they present on both internal and external displays, and if they are related to Dark Mode or Color correction issues, or a genuine hardware problem.

What is your indication that it is failing?

mypetduck wrote:

Can a power supply be failing and yet not be reported by running Apple Diagnostics?

The user Diagnostic...not as definitive as Apple's own AST2 (Apple Service Toolkit) back bench diagnostic.

In or out of warranty you can get a free over the counter 'Apple Service Diagnostics' test /assessment

Make an appointment for a "hardware issue"

Genius Bar Reservation and Apple Support Options - Apple

Outside the USA

https://locate.apple.com/country

Maybe if you paint a bigger picture surrounding the circumstances and some details—something can be added here

Old issue, new issue, what changed?

What exact Mac is this..?

iMac (Retina 5K, 27-inch, 2020) - Technical Specifications

In the Box

• iMac with 5K Retina display
• Magic Keyboard
• Magic Mouse 2
• Power cord

Call Customer Support (800) MY–APPLE (800–692–7753)

mypetduck wrote:

And the problem has reappeared.

In or out of warranty you can get a free over the counter 'Apple Service Diagnostics' test /assessment

Make an appointment for a "hardware issue"

Genius Bar Reservation and Apple Support Options - Apple

Outside the USA

https://locate.apple.com/country

Call Customer Support (800) MY–APPLE (800–692–7753)

or on line Apple Support

Learn how to get your Mac fixed and how much it will cost. Mac Repair Service Mac Repair - Official Apple Support

Get your Mac ready for service - Apple Support Get your Mac ready for service - Apple Support

PowerMyMac sounds like it is the culprit.

Use Reggie Answorth AppDelete to remove it by dragging the application over it.

Never install any optimizers, anti-virus utilities, or other utilities without verifying whether or not it is used for diagnostics or optimizing, and how to use it.

http://www.reggieashworth.com/appdelete.html

mypetduck wrote:

Thank you. What led you to that conclusion?

well, I will say it jumped out at me as well...

<PowerMyMac> - "A Smart Mac Cleaner to Clean up Mac."

Uninstall all third party apps that are Cleaners/Optimizers/VPN/Anti-Virus

all known to cause issues on the macOS

Look for an official uninstaller, stand alone, in app, About, Preferences, Package Contents, etc. if available—

 if in doubt search the developers website or contact their: Support/Help/FAQ

then compare your results

Many of us have come to that conclusion via many, many topics here in the Apple Support Communities regarding those types of apps and the damage they've done to systems and 3rd party apps.

There is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.