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Mid 2012 model, A1278 / 2.5 GHz i5 or 2.9 GHz i7 processor.

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Intermittent Temperature Sensor Failure

So, I bought a Macbook Pro 13 (non-retina) Mid 2012 i5. It was sold to me as needing a OS reinstall and possibly a new HDD. Turns out the HDD was fine - but I reinstalled the OS anyway on SSD, it's now running El Capitan. When the computer works, it works fine.

The problem is that sometimes, quite randomly in my opinion, some temperature sensors on the Macbook go crazy, showing 128C. Here is a screenshot from HWMonitor when this happens. Then the kernel_task usage goes up to 300% or more, the computer starts working slowly, and the fan revs up. I guess this is why the previous owner thought theres something wrong with the OS installation. After some time, maybe after 10-20 seconds, the sensors start showing normal temps again - first all other sensors except the Memory module ones - and fews secs later those as well - and the kernel_task cpu usage goes down.

When I was on Yosemite, I tried the trick on RDoxenham's site and it worked. But on El Capitan that can't be done anymore.

I have tried running the hardware diagnostics that can be accessed by pressing D on startup. I did that maybe 6 times, also the extended test. It never finds any trouble codes.

My question is; why exactly those sensors go AWOL? Can something be done?

Answered! View the answer I have this problem too

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I would start off with a better tool here to see what's happening. Download Temperature Gauge Pro it will allow you see track over time what is happening across the sensors and the fans.

The other tool which you already know about is Activity Monitor in the Utility folder. Here you can watch what process is consuming the CPU.

Often when I see this the hard drive SATA cable is breaking down. What happens is the drive and CPU are not able to pass data across without CRC errors so it resends the block and it does this over and over again so both the HD, CPU & System Hub Controller overheat. This problem won't happen with a light work load only when you have a lot moving about.

The other issue here is making sure your systems firmware is upto date. Make sure you are at what this Apple T/N has or newer: About EFI and SMC firmware updates for Intel-based Mac computers.

Post a screenshot of TG Pro as well as the log file so we can be sure we're on the right track here.

Update (10/07/2015)

If you follow the log you can see time points the CPU gets hot and when it does the TGP is reporting the CPU sensors are not reporting. So you have in fact have two issues:

  • The first is the ramp up of the temp leads me to believe the SATA cable is bad so you do need to replace it
  • The second here is the logic board needs some work. It maybe as simple as a bad solder joint or a SMT component that is failing within the CPU logic area that reports to SMC.

But for now focus on the replacing the SATA cable and see if the temp issue goes down if it does you maybe able to live with the sensor issue. If not, this is a board repair or time to replace the logic board.

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Can you give us the make and model of the SSD you put in. Also did you enable TRIM services with in El Capitan?

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Here is a Pastebin copy of a shortish log file, i just installed TGP.

http://pastebin.com/tEMi0NU6

Here is the screenshot of TGP, everything was working fine then:

http://imgur.com/ViUWfeQ

(The computer has now the regular HDD again, I had to put it back because the SSD went to my old laptop I sold today). The firmware is up-to-date.

Just for kicks, I ran Linux Mint today off of USB, so the HDD was not used at all, not even mounted - and all the sensors showed normal figures all the time. I kept it running for few hours, browsing the web etc. That makes me think the SATA cable thing you mentioned makes actually a lot of sense.

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Thanks for your insightful answer. I will order a new sata cable and replace it. I won't be replacing the logic board because the laptop is out of warranty and getting old, i'll live with the not working cpu sensor. Thank you very much for your help.

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Hope it all works out. If the temp still is high you might want to make sure the heat sink & fan is clean as well as look at refreshing the thermal paste.

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Travis Bickle will be eternally grateful.
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