TLDR
When I upgraded the fusion drive to SSD, I opted to include the OWC in-line Digital Thermal Sensor. That sensor was the root cause of the repair needed here, and ironically, I don’t think it was ever needed in this model iMac.
Detail
A couple of years ago, I removed the 1Tb fusion HDD from my iMac 2019, to replace with a 2Tb Crucial MX500 SSD. There was some confusion at the time as to whether the iMac would depend on the drive having a thermal sensor, without which it may run the fan at full speed always. Therefore I got hold of the OWC bundle including Digital Thermal Sensor and Glue strips and installed as written up here.
After just over 2 years of faultless operation there was a loud ‘crack’ noise from behind the screen and the iMac was instantly dead – with no means of powering it up. I was pretty convinced that the power supply board was the affected component and ordered a replacement (2nd hand) via ebay, and avoided opening up the iMac until that had arrived.
I then started work cutting the screen glue with the old iFixit cutter. It felt much more difficult than the original glue a couple of years ago, and at some point I managed to break the cutter wheel.
At which point I ordered a new cutter with spare blades via Amazon. I then made a crutial mistake… I attempted to continue working around the screen but using a pick tool. I should have just been patient and waited for the next day delivery, as the pick tool caused extra stress on the screen resulting in a chip of glass breaking free and some hairline cracks. Fortunately the worst of it is in the black bevel are, and I have simply covered with black electrical tape.
With the replacement blades the next day, I was able to remove the screen. At this point the root cause of the problem was immediately apparent. As you can see from the photo of my original work, the OWC sensor is stuck to the SSD.
With the screen removed, we can see that the sensor is nolonger stuck to the SSD, and has dropped, with the wires just long enough for the sensor to land on the power supply board. Closer inspection shows soot on the sensor board and missing components, and against one of the transformers of the power supply. The back of the sensor had lost all stick – the glue has completely denatured.
I did have some worry about the power supply board that I had obtained via eBay – it was from an iMac 5K 2020, and while it looked the same size, its components were in a different layout. It does turn out that the board works well in this iMac.
However, once installed, it was apparent that the SSD was no-longer functioning. So the next step, I ordered a Crucial 2Tb BX500. This turns out to be a potential mistake. On a cold boot, the Mac shows the prohibited icon for a while. This eventually clears, but the drive performance, particularly in the time machine restore is very poor.
In consultation with Claude.ai, the suggestion is that the BX500 is a budget drive and has some Mac compatibility issues. However following a few weeks basic operation which only involves creating and editing a few lightweight documents and online streaming, I have not hit any noticable issues – other than on a cold boot and that ‘prohibited’ icon.
The fan has been running normally, therefore I do not believe the thermal sensor was actually necessary in the first place, and my belt and braces approach has turned out to increase rather than decrease risk on this occasion.
If you decide to install the sensor (or already have) i recommend using cable ties and routing the sensor wires such that when it comes unstuck, it cannot fall into the power supply, or touch any other metal component.
