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Model A1419 / Late 2013 / 3.2 & 3.4 GHz Core i5 or 3.5 GHz Core i7 Processor, ID iMac14,2

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Bad SMC, CPU socket, sensors or logic board?

I recently replaced my late 2013 27" iMac logic board with a used logic board I got for cheap on eBay. The logic board was sold as fully working and pulled from a working unit, however the price seemed to good to be true (40 USD!).

When I got the replacement board, I noticed that several pins in the CPU socket were bent out of place. I spent an hour fixing them using a microscope to actually see what I was doing. When I was done, every pin seemed to be back where it belonged. After installing the board, it booted up fine, however the system was heavily throttled and the fan was constantly running full speed.

I took care of the fan speed with macfancontrol (which reports temperatures from all sensors normally) and of the throttling with OCLP. I already tried replacing the LCD temperature sensor, making sure that the system firmware is the latest and installing the original HDD, but without software patching, the system still acts up.

It's funny how whenever I plug the iMac into power, the breaker in my apartment goes out, but after restoring power, the iMac will boot up and work just fine (with the patches), going as far as pulling through several stress tests without any issues.

I'm pretty sure that the issue is logic board related and not PSU related, as the machine always worked fine with the original logic board until the point the GPU failed. Running apple diagnostics I got error PFM006 "there may be an issue with the SMC".

I'm curious to know if anyone had a similar experience or has any suggestions on how to fix this issue. I have some experience micro soldering dealing with small SMD components, however I don't have access to schematics and I don't think I'm comfortable working on a big BGA chip like the SMC, considering that the board is still kinda usable for the time being.

Update (02/03/24)

These sensors may not be working properly: unknown name

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Update (02/05/24)

After replacing Q5575 (DIMM B proximity sensor) the problem was completely fixed!

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Sounds like the cord or power supply has a short or the outlet you are using is mis-wired (common and hot are reversed and the ground (neutral) is tied in the panel to the common creating a ground loop condition!

Time to breakout the cheap outlet checker to test your outlets (as many as you can) as you could have another device creating the problem as well with a bad cord.

You will also want to check your breaker panel (may want to get an electrician to help you here) to verify the common and neutral sides are correctly wired and are not hot. Then check the buildings ground is in good working order as well.

As to your system

I would run the onboard diagnostics again to see if it reports any other errors than the generic PFM006. While it will catch many it’s not complete! Let us know what it reports back.

Next, I would install a good thermal monitoring app like TG Pro I find it the most useful. The full version offers a bit more help in diagnosing fan and some sensor issues than trying to force things with macfancontrol

PFM006 Error is related to the SMC logic but not necessarily the SMC chip it’s self. Here it could be a thermal or power sensor within the logic board that is either not working or reporting the wrong values.

It’s likely this logic board has deeper issues. Why did you replace yours?

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Thank you for your reply.

As I stated, I replaced my board due to a faulty GPU.

I tried a different AC cable and nothing changed. I should add that the breaker doesn't go out every single time I connect the iMac to power, "only" 90% of the times (rough estimate).

As the building I live in was recently sold to a new owner, an electrician actually came to check the breaker panel and every single outlet in the apartment less than 2 months ago and everything seemed to be perfectly normal.

As you suggested, I installed TG Pro, at the bottom it does say that the "unknown sensor" may be faulty, however this sensor is not displayed anywhere in the app. I did notice that the proximity sensor for memory DIMM B is completely missing from both TG Pro and macfancontrol, I'm not sure if this is normal or if this missing sensor is the culprit here. (See screenshot).

I ran the onboard diagnostics once more and got the same error again (PFM006).

par

@del_b266 - I’ll need to review the schematics to see what’s up here.

par

@danj I was finally able to source a boardview and schematics myself. It looks like there are indeed 4 different thermal sensor, one for each memory slot. The fact that one is completely missing from TG Pro seems wrong, so I guess that means I'll have to remove the board and inspect the area

par

@del_b266 - You beat me😊

Yep, time to get dirty. You’ll need to trace the lines from the sensor to SMC looking for a cut trace (corrosion damage) or likely a bad solder joint or damaged SMD device (resistor or capacitor). Did you grab a board view as you’ll need that as well.

par

@danj Mission accomplished!! THANK YOU SO MUCH for helping me diagnosing the issue!! It turns out that Q5575 was completely missing from the board, so I harvested it from the original board with the dead GPU and installed it onto the replacement board. I also replaced L6307 as it was chipped. After reassembly, the iMac booted up just fine, but the loud fan noise was gone. The DIMM B proximity sensor appeared in TG PRO and after running AHT I now get a ADP000 - no problems detected. The other day I contacted the eBay seller complaining about the issues with the board, particularly the bent socket pins. I got no reply, but a refund for the board was issued (but not for international shipping, which was almost as much as the board)... I guess all is well that ends well.

par

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b266 sera éternellement reconnaissant.
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