In order to enable a fan in Marlin that is controlled automatically based on the extruder temperature (read: If the extruder is hot, turn on the fan), open Configuration_adv.h and look for the Extruder cooling fans section and set for example E0_AUTO_FAN_PIN to whatever pin you want to be automatically controlled.
/** * Extruder cooling fans * * Extruder auto fans automatically turn on when their extruders' * temperatures go above EXTRUDER_AUTO_FAN_TEMPERATURE. * * Your board's pins file specifies the recommended pins. Override those here * or set to -1 to disable completely. * * Multiple extruders can be assigned to the same pin in which case * the fan will turn on when any selected extruder is above the threshold. */ #define E0_AUTO_FAN_PIN PA_8 #define E1_AUTO_FAN_PIN -1 #define E2_AUTO_FAN_PIN -1 #define E3_AUTO_FAN_PIN -1 #define E4_AUTO_FAN_PIN -1 #define E5_AUTO_FAN_PIN -1 #define E6_AUTO_FAN_PIN -1 #define E7_AUTO_FAN_PIN -1 #define CHAMBER_AUTO_FAN_PIN -1 #define COOLER_AUTO_FAN_PIN -1 #define COOLER_FAN_PIN -1