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Where exactly is CPU temperature being measured?

As noted in the section on monitoring main sensors, the individual sensor equipment of Macintosh computers can be very different depending on model type. To ensure ease-of-use, System Monitor automatically selects a certain sensor as being typical for representing processor temperature, and this sensor can be shown in the menu bar. The following policy is used to determine this sensor:

  1. A temperature sensor as close as possible to the processor chip will be selected. Sensors within the processor cores won’t be considered due to technical reasons, however.
  2. In case more than one processor chip is available, the first processor will be selected. For Macintosh models with numeric processor designations, this will be processor 0. Otherwise, the processor with designation A will be selected.
  3. If available, the temperature diode of this processor will be selected, if not, a sensor at the heat sink of this processor. In case this sensor is unavailable as well, the proximity sensor of this processor will be used.