Intel E6420 - Core 2 Duo Dual-Core Processor Datasheet page 109

Boxed Processor Specifications
If the new 4-pin active fan heat sink solution is connected to an older 3-pin
baseboard processor fan header it will default back to a thermistor controlled mode,
allowing compatibility with existing 3-pin baseboard designs. Under thermistor
controlled mode, the fan RPM is automatically varied based on the Tinlet temperature
measured by a thermistor located at the fan inlet.
For more details on specific motherboard requirements for 4-wire based fan speed
control, refer to the appropriate Thermal and Mechanical Design Guidelines (see
Section 1.2).
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