iMac 24" (Early 2009) random shutdown

Question

I inherited a friend's iMac 24" (Early 2009) for free. So putting a few bucks and time was worth refreshing a old machine for my 75 year old mom.

I started with stock machine (HDD, 4 GB RAM, El Capitan). It was super slow but worked. I maxed 8 GB RAM, swapped to 500 GB SSD, and reinstalled fresh El Capitan. (Side note: pain in the ass. Must start with macOS Leopard, and work your way up to El Capitan).

Anyway, along the way of all these OS updates the machine would just randomly shut down. Sometime in first 10 minutes or 40 minutes, or a day later.

As I could not find the issue, I tried installing OpenCore Legacy Patcher to Monterey. All went smooth and the machine seemed quicker and runs great. Although the random shutdown still haunts me.

It’s totally random as time, open apps or not, etc. Any suggestions?

Answer 1

Do you have the original installer disk set? If so you could run the Apple Hardware Test [AHT] from there. Hold D at the chimes.

If not, you may have some success running AHT from a USB stick - an 09 iMac ought to be able to boot to AHT from USB, if the OpenCore Patcher didn't upset it ;)

It's probably sensible to blow the NVRAM & reset SMC first, just in case, but I don't expect it will make any difference.
SMC is unplug from power for a minute, long-press the power button [just to be sure it power-soaked]. Plug back into mains, wait 15s, boot.
NVRAM is hold Cmd/Opt/P/R at the chimes & wait for it to reboot again.

There is a list of AHT files at https://github.com/upekkha/AppleHardwareTest - the files come directly from Apple, so are safe. The list of links is just to make them possible to find.
You need to know the exact Mac model & firmware version - the page gives instructions on how to determine that information & build the USB boot drive. I assume as you managed to get Monterey running, this is not going to be too much of a challenge.

There's a list of AHT error codes at LogiWiki - Apple Hardware Test (AHT) and Apple Diagnostics (AD) Codes if it does find anything wrong.