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The larger of Apple's MacBook Air laptops featuring dual microphones and 802.11ac Wi-Fi connectivity.

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What is a non-obvious cause for a faulty temperature sensor?

My MacBook Air suffers from intermittent slowdowns and unnecessary fan use. I sent it to Apple under warranty and they replaced the battery and the logic board. But the problem persists and iStats shows that the temperature is reading -128 C.

It seems that Apple was right to replace the logic board which includes the temperature sensor (right?) But, why didn't that fix the problem? It seems very unlikely to have two faulty logic boards, both with bad sensors.

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Run ASD, tell me what temperature sensor fails. There are many temperature sensors. One in the trackpad, one on DC in board, ones on the board itself. Sometimes the sensor fails, sometimes the sensor is fine but cannot communicate with the SMC because of bad or missing pullup resistors on a data line, corroded via, corrosion pulling down a data line, or a bad SMC.

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ASD? Apple System Diagnostics? I get codes PFM006 (fan), PPN001, and NDC001 (camera). Not sure how to to determine which thermal sensor is bad.

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Louis you're right. Apparently there are also three sensors on the battery. TB0T, TB1T, TB2T.

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