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対象のモデルはA1419 / EMC 2806 / Late 2014 または Mid 2015。 3.3 または 3.5GHz Core i5 または 4.0GHz Core i7 (機種ID iMac15,1)、EMC 2834 late 2015 / 3.3 または 3.5 GHz Core i5 または 4.0 GHz Core i7 (機種ID iMac17,1)のRetina 5Kディスプレイを搭載したすべて。

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Fan at max speed, PSU thermal sensor shows no readout

Since yesterday, my iMac (A1419 EMC 2806) fan has been maxing out from power up, regardless of safe mode, recovery mode, or booting into Windows (via Boot Camp).

Diagnostics return error PFM006

Assuming a thermal sensor defect, I installed iStats to check the thermal sensors. Readout below in the screenshot - all temperatures are within normal ranges, the fan is at maximum and also won't slow down if I try to do that manually, and the only temperature that's missing is the one from the power supply unit (AC/DC T2 secondary heat sink).

So my question is: Is there even such a sensor in that model, do I expect a readout here? Have I thus found the probable cause and should be looking into the connection or the sensor itself, or the power supply unit (I heard the sensor is integrated into it, when it exists)? No other hardware or software changes made recently.

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Update (10/16/2021)

Have to revise myself, I tried it without the display connected and the fan went full speed, but I forgot that the LCD features thermal sensors for itself that weren't connected then!

When I connected the display, the fan seemed off, but now I broke the connector cable and I have to wait for a new one :-( so maybe it's solved, I'll update...

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Update (10/20/2021)

After exchanging my damaged display cable and bending the connector back to get it to work, I can now confirm that after exchanging the power supply unit as a whole, everything works perfectly! No fan powering up anymore, and iStats shows a normal temperature value for the AC/DC T2 sec. heatsink (I'll post a screenshot later, just have to restore the OS first as after doing a quick check, before reassembling everything I used the opportunity to upgrade to an SSD and a better CPU)! So yes, it was the power supply, and you have to exchange it as a whole indeed! Sadly,I couldn't locate the sensor on the power supply, it'd be a shame if you couldn't solder on just a new one!

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I would run the onboard diagnostics to see what it tells you. Restart your system and press the D key

Reference: Mac startup key combinations

Did you get any errors?

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See above: diagnostics return error code PFM006.

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Can confirm, yes there is such a sensor in this model. And yes it is built into the power supply and not onto the logic board itself.

So if cleaning the unit out doesn't help, try reseating the signal cable from the power supply to the logic board (it’s the sort of short squiggly one on the front of the PSU that goes to the logic board, can’t miss it). This would carry SMC related data from the power supply. It seems unlikely it would be bad since it’s also the cable that carries data from the Power button, and that’s working, so unless it’s specifically the SMC line on the cable that’s a problem, I doubt this is the issue.

The other Power Supply connector around the back of the board is only for 12V power, no data at all so no use trying to fuss with that in my opinion since it’s a pain. It’s a tight connection, on a short cable, in a tight area with a lot of exposed, large capacitors in close proximity.

If reseating the data cable doesn’t fix the issue, then I would say a new PSU is the way to go.

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I exchanged the power supply unit as a whole, and then it worked perfectly! No fan powering up anymore, and iStats shows a normal temperature value for the AC/DC T2 sec. heatsink! So yes, it was the power supply, and you have to exchange it as a whole indeed!

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Sorry I had missed you stating the error code PFM006. A single text blob makes it hard to find things.

Yes your system has such a callout, jump to iMac15,1 in this listing SMC_Sensor_Codes

Key Tp2h - low 0 - high 0 - AC/DC T2 Sec Heatsink

But just because its present doesn’t mean it’s actively being used and not all 3rd party tools offer enough info diagnostically. I’ve found the full version of TG Pro offers a bit more. For 10 bucks its worth a try.

I think its time to open the system up and at least give the system a good clean of dust buildup using a soft small paint brush and a can of can’ed air. Cleaning the fan blades and the heat sink fin area. Review this guide iMac Intel 27インチ Retina 5K Display ロジックボードの交換 make sure you use the pizza tool in the iMac Intel 27" (2012-2019) Adhesive Strips and get two sets of Plastic Cards so you can remove the display without damaging it. Be sure you can do this other wise you may want to bring your system to a independent Mac repair shop as they will repair your logic board Vs telling you they need to replace it which is what the Apple Store will tell you.

I’m hoping you will either see a loose connection or something damaged (burnt component) are you able to micro solder?

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Plastic Cards

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iMac Intel 27" (2012-2019) Adhesive Stripsの画像

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iMac Intel 27" (2012-2019) Adhesive Strips

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Yes, I can micro solder, and I had thought of a similar approach to this. I'll get the opening tools and I'll hope for something obvious to be found, but I'll also check TG Pro first. I'll check back here when I find out more.

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TG Pro shows all sensors within normal temp range, but the AC/DC T2 sec. heatsink sensor doesn't appear among them, and it also gives the vague error message: "This sensors may not be working properly: Unknown name", which kinda doesn't let me pinpoint the problem, but I'd say this still hints at the PSU temp sensor. I ordered the opening kit and I'll open it up the next couple of days, let's see what we find there. (Link to TG Pro screenshot: https://i.postimg.cc/yxVVbHg9/Bildschirm...)

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the only info I have on that sensor is what I found here, so I'll focus on that area Sensor error Tp2H, heatpipe, power supply

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and another thing I noticed: TG Pro reads out a "DC in proximity" sensor with a normal temperature. Could be that that's actually the one that had no readout in iStats? also, TG Pro reads out 5 temperatures more, but I couldn't fully evaulate the lists and compare them to your (BTW great!) LogiWiki, didn't want to waste the time here when I have to open it up anyway. Many are named completely differently and it's hard to figure out which ones are actually the same. Just saying, for those encountering similar problems.

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@Cirkmann Zirkel The DC in proximity sensor would be on the logic board, and on your original sceenshot with temps would be the AC/DC 12V G3H sensor. I definitely suspect your bad sensor is the one in the Power Supply.

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Cirkmann Zirkel さん、ありがとうございました！
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