Chosen Solution

So i tried to replace my i5 2400 3,1 GHZ CPU with a i7 3770 it did not work. I reinstalled the i5 2400 and everything worked normal except the Fans for ODD and CPU. Well i thought no problem i will regulate it with software. But then i experienced a big drop of perfomance in OBS and Logic. I installed intel Power Gadget and noticed the CPU Frequency is locked to 1.6GHZ. The Apple Hardware-Test says: 4SNS/1/C0000008: TLOp–124 I searched in the Web and found that it must be a Temperature Sensor connected to the LCD.

  1. SMC PRAM VRAM Reset
  2. I ordered a new LCD Tempsensor - replaced it but nothing changed.
  3. I ordered a new V-SYNC /TempSens Cable (connected to the LED Backlightboard) nothing changed.
  4. I ordered a new LCD Data Cable - nothing changed.
  5. I ordered a new Logic Board + CPU - nothing changed. So these are the steps I did until now and I don’t know what to do now… The only thing left i could imagine would be the LED-Backlightboard maybe reinstall OS or could be something wrong with the EFI, Powermangament I dont know.. Maybe someone experienced the same issue or knows how to repair it. Everything else except Fans going wild and CPU is throttled works well.

EDIT: (This Programm is called HW-Monitor and is the only one that shows me 128° on LCD)

EDIT: So i was away for like 2 hours and the imac was in Sleepmode when i came back i saw this:

Sadly it went back to 1.6GHZ after a short period of time. EDIT: So its definely reproducable. I have to put the iMac in Sleepmode for 20-30secs and when i wake it up the CPU runs normal ant turns to 1.6GHZ after 10secs. What the !&&*… Maybe another hint. When I turn on Sleepmode the fans are stopping. NOW when i wake up the MAC TGPro is showing my some N/A Temperatures: GPU DIODE Platform Controller Hub DIe As soon as these both Temperatures are availabe the Fans power up and the CPU gets throttled.

EDIT: Tried to reinstall OS X did not help.. EDIT: Tried to run the iMac without the LCD on a second Monitor did not work. When i opened the iMac i noticed strange “glue” on the poweradapter:

Also i run a SystemDiagnosis which gave me the same error as the AHT:

This week i will probably bring it to an Authorized Apple Reseller and hope the can help me.

I was able to fix the problem. Switched the LED-Backlightboard and it worked.. I think the connector to the LCD V-Sync/temp - cable was faulty. Thanks for everyone considering solutions!

Can’t say for variance, but it sounds like some temp sensor for the CPU is going crazy. The high fan speed and throttling could be because of thermal reasons (or so it thinks). Try reseating the CPU and see if that helps. Also, tell your temps please. EDIT: did you replace the thermal paste?

I’m having the same problem with clock speed (blocked at 1,6Ghz) and fans at full speed BUT Apple Hardware-Test is not giving me any error or failure. I’ve passed AHT a lot of times with no errors detected, every sensor is giving corrects values,… i don’t know from where is coming the issue, but the symptoms are the same. Something is happening with SMC that Apple Hardware-Test is not seeing. The computer is an 2011 iMac i7 at 3,4Ghz.