Be consistent in setting of the fan PWM.

Always set the FAN PWM value in check_axes_activity() and always use
the value of the active move if a move is active.

Marlin/planner.cpp

   unsigned char y_active = 0;  
   unsigned char z_active = 0;
   unsigned char e_active = 0;
-  unsigned char fan_speed = 0;
-  unsigned char tail_fan_speed = 0;
+  unsigned char tail_fan_speed = fanSpeed;
   block_t *block;
 
   if(block_buffer_tail != block_buffer_head)
       if(block->steps_y != 0) y_active++;
       if(block->steps_z != 0) z_active++;
       if(block->steps_e != 0) e_active++;
-      if(block->fan_speed != 0) fan_speed++;
       block_index = (block_index+1) & (BLOCK_BUFFER_SIZE - 1);
     }
   }
-  else
-  {
-    #if FAN_PIN > -1
-    #ifndef FAN_SOFT_PWM
-    if (fanSpeed != 0){
-      analogWrite(FAN_PIN,fanSpeed); // If buffer is empty use current fan speed
-    }
-    #endif
-	#endif
-  }
   if((DISABLE_X) && (x_active == 0)) disable_x();
   if((DISABLE_Y) && (y_active == 0)) disable_y();
   if((DISABLE_Z) && (z_active == 0)) disable_z();
   }
 #if FAN_PIN > -1
   #ifndef FAN_SOFT_PWM
-  if((fanSpeed == 0) && (fan_speed ==0))
-  {
-    analogWrite(FAN_PIN, 0);
-  }
-
-  if (fanSpeed != 0 && tail_fan_speed !=0)
-  {
-    analogWrite(FAN_PIN,tail_fan_speed);
-  }
+  analogWrite(FAN_PIN,tail_fan_speed);
   #endif
 #endif
 #ifdef AUTOTEMP

Marlin/ultralcd.cpp

     setTargetHotend1(plaPreheatHotendTemp);
     setTargetHotend2(plaPreheatHotendTemp);
     setTargetBed(plaPreheatHPBTemp);
-#if FAN_PIN > -1
     fanSpeed = plaPreheatFanSpeed;
-    analogWrite(FAN_PIN,  fanSpeed);
-#endif
     lcd_return_to_status();
 }
 
     setTargetHotend1(absPreheatHotendTemp);
     setTargetHotend2(absPreheatHotendTemp);
     setTargetBed(absPreheatHPBTemp);
-#if FAN_PIN > -1
     fanSpeed = absPreheatFanSpeed;
-    analogWrite(FAN_PIN,  fanSpeed);
-#endif
     lcd_return_to_status();
 }