2.2 GHz (Turbo Boost up to 3.4 GHz), 2.5 GHz (Turbo Boost up to 3.7 GHz), or 2.8 GHz (Turbo Boost up to 4.0 GHz) quad-core Intel Core i7 processor with 6 MB shared L3 cache.

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Do the fans work in conjunction or are they separate GPU/CPU fans?

Do both the fans in a mid 2015 MBPr with a discrete AMD GPU work in conjunction, or is one the CPU fan and one the GPU fan?

If so, which fan does which?

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Each fan is controlled discretely based on what is in need of cooling.

You can see this with a good temperature monitoring app like TG Pro.

If you suspect you are having a problem I would encourage you to install this app and post a screenshot of what it shows (sensors & fan) so we can try to figure out what's wrong.

Given the age of the system it maybe covered under warranty.

Update (11/11/2015)

Download the App I pointed out to you it will clarify things.

The dual heat sink model has the heat sinks tied together by a common cooling bar to the two fan unit radiators which allows them to share some of the heat load from the discrete CPU & GPU.

Why are we picking this apart, you can't alter the design nor should you.

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There's no problem - I was curious about how the fans are controlled.

Do you know if each fan works in response to a particular sensor? e.g. is the left fan responding to CPU temp and the right fan to GPU temp, or are they both governed by the average temp in the case?

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It depends on which model you have. Apple has two versions: The differences between the 15-Inch "Mid-2015" Retina Display MacBook Pro models The dual GPU based systems have two heat sinks tied together via a heat pipe. The fan unit nearest the discrete GPU will be biased when the discrete GPU's thermal sensor is set off and the reverse is true for the CPU. In addition the heat sinks own sensor will also set the CPU in power down mode until the temp is lowered which also effects the discrete GPU. The intergraded GPU models ramp up/down both fans in unison as it only has one heat sink which is piped to both fan units.

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Sorry, I seem to be really bad at clearly asking the questions I wanted answered.

The left fan has a higher maximum RPM than the right fan, so what I wanted to know was if the fans operate independently? (i.e. in a hypothetical scenario were the CPU heatsink is hot and the GPU heatsink is cool is one fan fast and one slow?)

I've not been able to locate anywhere that says if the two fans are linked by a heat pipe (the difference in RPM leads me to guess that they're not) or if they're operating against discrete heat sinks.

If so:

Does the left fan or the right fan respond to the temperature of the GPU heat sink?

Does the left fan or the right fan respond to the temperature of the CPU heat sink?

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