Where to enable extruder-temperature controlled fans in Marlin?
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