This fan is the smaller of the two fans mounted in front of the extruder and is also used inside the electronics case.
These fans have a keyed connector, guarding against incorrect plug-in. All fans should be kept free of dust buildup to prevent failure.
*M2 3D printers purchased in November 2015 or earlier require the 12 volt version of the 40 mm fan—please be sure to select the proper voltage based on your date of purchase. If you are unsure which fan you need, you can examine the label on your current fan to verify its voltage.