Released October 24, 2011 / 2.2, 2.4, or 2.5 GHz Quad-core Intel Core i7 Processor

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Is the sensor reading the fan speed located in the fan itself?

My left fan is working full speed (goes down to normal speed for few secs) . After lots of troubleshooting, I deduced that the computer is not able to read the proper RPM of the left fan (very low 0-200 RPM), hence it operates the fan at the highest speed as a fail-safe precaution.

Now I am wondering, if this reading failure is stemming from a logic board issue or fan issue.

So where is the sensor located? Is it in the fan itself or on the logic board?

I ran AHT and it returned 4MOT/4/40000002 left side -0

I am asking to check if I am screwed or not :), and to decide whether to buy a new fan !!

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UPDATE: Disconnecting the fan from the logic board and reconnecting it again solved the issue. The fan has been normal for a week now.

I hope no one faces this issue, but if you do, try this before buying a new fan !!

Answered! View the answer I have this problem too

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You'll need to replace the fan unit. The sensor is within the fan and not serviceable. Here's the IFIXIT guide: MacBook Pro 15" Unibody Late 2011 Left Fan Replacement and heres the part you'll need: Left Fan unit - Apple P/N 661-4952

MacBook Pro 15" Late 2008 through Mid 2012 (excluding Mid 2009 2.53 GHz) Left Fan Image

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Comments:

Thanks a lot, Dan. I will buy a new fan and install it. I am very optimistic that a replacement would solve the issue, if the sensor is within the fan unit.

If you don't mind my asking, this fan is quite expensive compared to other CAD $15-$30 fans found on Amazon and takes a long time to reach Canada. In terms of quality & reliability, do you think I should stick to this unit (the one you shared), or go with those on Amazon?. I am asking because they don't have reviews and I am afraid they might fail very soon, if they have a poor-build quality.

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Sadly, you do take chances on Amazon. The fan maybe from a dead system and not tested. So do make sure you pay with a credit card that offers some purchase protection.

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Thanks, Dan. I appreciate it

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