Check the temperature before filament change

Marlin/Marlin_main.cpp

    */
   inline void gcode_M600() {
     float target[NUM_AXIS], lastpos[NUM_AXIS], fr60 = feedrate / 60;
+    if (degHotend(active_extruder) < extrude_min_temp) {
+      SERIAL_ERROR_START;
+      SERIAL_ERRORLNPGM(MSG_TOO_COLD_FOR_M600);
+      return;
+    }
+
     for (int i=0; i<NUM_AXIS; i++)
       target[i] = lastpos[i] = current_position[i];
 

Marlin/configurator/config/language.h

 #define MSG_ENDSTOPS_HIT                    "endstops hit: "
 #define MSG_ERR_COLD_EXTRUDE_STOP           " cold extrusion prevented"
 #define MSG_ERR_LONG_EXTRUDE_STOP           " too long extrusion prevented"
+#define MSG_TOO_COLD_FOR_M600               "M600 Hotend too cold to change filament"
 #define MSG_BABYSTEPPING_X                  "Babystepping X"
 #define MSG_BABYSTEPPING_Y                  "Babystepping Y"
 #define MSG_BABYSTEPPING_Z                  "Babystepping Z"

Marlin/language.h

 #define MSG_ENDSTOPS_HIT                    "endstops hit: "
 #define MSG_ERR_COLD_EXTRUDE_STOP           " cold extrusion prevented"
 #define MSG_ERR_LONG_EXTRUDE_STOP           " too long extrusion prevented"
+#define MSG_TOO_COLD_FOR_M600               "M600 Hotend too cold to change filament"
 #define MSG_BABYSTEPPING_X                  "Babystepping X"
 #define MSG_BABYSTEPPING_Y                  "Babystepping Y"
 #define MSG_BABYSTEPPING_Z                  "Babystepping Z"