The Sensors menu extra monitors your temperatures, fans and various power sensors. If you’d like to hide a sensor, you can choose Hide This Sensor from the submenu on every sensor in the sensor dropdown menu.
To reset all hidden sensors, choose Reset Sensor Filtering from the Stats menu in iStat Menus.
If you’re having trouble controlling your fans on an Intel Mac, it’s possible you may need to reset your System Management Controller (SMC). For more information, please read Apple’s instructions on resetting your SMC.