Finally, all seems to work as expected. Eureka!

MK/module/MK_Main.cpp

@@ -42,6 +42,10 @@
   CardReader card;
 #endif
 
+#if ENABLED(FLOWMETER_SENSOR) && ENABLED(MINFLOW_PROTECTION)
+  bool flow_firstread = false;
+#endif
+
 bool Running = true;
 bool Printing = false;
 
@@ -749,6 +753,9 @@ void setup() {
   #endif
 
   #if ENABLED(FLOWMETER_SENSOR)
+    #if ENABLED(MINFLOW_PROTECTION)
+      flow_firstread=false;
+    #endif
     flow_init();
   #endif
 
@@ -2969,8 +2976,14 @@ inline void do_blocking_move_to_z(float z) { do_blocking_move_to(current_positio
 
 #if ENABLED(FLOWMETER_SENSOR)
   void print_flowratestates() {
-    ECHO_MV(" FLOW: ", get_flowrate(), 1);
-    ECHO_M(" ml/min ");
+    float readval;
+    readval = get_flowrate();
+    #if ENABLED(MINFLOW_PROTECTION)
+    if(readval > MINFLOW_PROTECTION)
+      flow_firstread=true;
+    #endif
+    ECHO_MV(" FLOW: ", readval);
+    ECHO_M(" l/min ");
   }
 #endif
 
@@ -9252,14 +9265,11 @@ void manage_inactivity(bool ignore_stepper_queue/*=false*/) {
   if (max_inactive_time && ELAPSED(ms, previous_cmd_ms + max_inactive_time)) kill(PSTR(MSG_KILLED));
 
   #if ENABLED(FLOWMETER_SENSOR) && ENABLED(MINFLOW_PROTECTION)
-    if (get_flowrate() < (MINFLOW_PROTECTION*1000)) {
-      if (Printing) 
+    if (flow_firstread && Printing && (get_flowrate() < (float)MINFLOW_PROTECTION)) {
+        flow_firstread = false;
         kill(PSTR(MSG_KILLED));  
-      //else
-      // stop();
     }
   #endif
-
   if (stepper_inactive_time && ELAPSED(ms, previous_cmd_ms + stepper_inactive_time)
       && !ignore_stepper_queue && !blocks_queued()) {
     #if DISABLE_X == true
@@ -9480,6 +9490,9 @@ void kill(const char* lcd_msg) {
   #if ENABLED(KILL_METHOD) && KILL_METHOD == 1
     HAL::resetHardware();
   #endif
+  #if ENABLED(FLOWMETER_SENSOR) && ENABLED(MINFLOW_PROTECTION)
+    flow_firstread=false;
+  #endif
 
   #if ENABLED(ULTRA_LCD)
     lcd_setalertstatuspgm(lcd_msg);
@@ -9593,6 +9606,9 @@ void kill(const char* lcd_msg) {
 #endif // FAST_PWM_FAN
 
 void stop() {
+  #if ENABLED(FLOWMETER_SENSOR) && ENABLED(MINFLOW_PROTECTION)
+    flow_firstread=false;
+  #endif
   disable_all_heaters();
   disable_all_coolers();
   #ifdef LASER

MK/module/flowmeter/flowmeter.cpp

@@ -23,18 +23,16 @@
 
 #if ENABLED(FLOWMETER_SENSOR)
 
-volatile byte flowrate_pulsecount;  
+volatile int flowrate_pulsecount;  
 float flowrate;
-unsigned int flowml;
 static millis_t flowmeter_timer = 0;
-
+static millis_t lastflow = 0;
 void flowrate_pulsecounter();
 
 void flow_init() {
 
    flowrate = 0;
    flowrate_pulsecount = 0;
-   flowml = 0;
    pinMode(FLOWMETER_PIN, INPUT);
    
    attachInterrupt(digitalPinToInterrupt(FLOWMETER_PIN), flowrate_pulsecounter, FALLING);
@@ -45,18 +43,25 @@ void flowrate_manage() {
    now = millis();
    if(ELAPSED(now, flowmeter_timer)) {
       detachInterrupt(digitalPinToInterrupt(FLOWMETER_PIN));
-      flowrate  = ((1000.0 / (now - flowmeter_timer)) * flowrate_pulsecount) / FLOWMETER_CALIBRATION;
+      flowrate  = (float)(((1000.0 / (float)((float)now - (float)lastflow)) * (float)flowrate_pulsecount) / (float)FLOWMETER_CALIBRATION);
+      #if ENABLED(FLOWMETER_DEBUG)
+         ECHO_M(" FLOWMETER DEBUG ");
+         ECHO_MV(" flowrate:", flowrate);
+         ECHO_MV(" flowrate_pulsecount:", flowrate_pulsecount);
+         ECHO_MV(" CALIBRATION:", FLOWMETER_CALIBRATION);
+         ECHO_E;
+      #endif
       flowmeter_timer = now + 1000UL;
-      flowml = (flowrate / 60.0) * 1000;
-
+      lastflow = now;
       flowrate_pulsecount = 0;
       attachInterrupt(digitalPinToInterrupt(FLOWMETER_PIN), flowrate_pulsecounter, FALLING);
+
    }
 
 }
 
-int get_flowrate() {
-   return flowml;
+float get_flowrate() {
+   return flowrate;
 }
 
 void flowrate_pulsecounter()

MK/module/flowmeter/flowmeter.h

@@ -24,7 +24,7 @@
 #if ENABLED(FLOWMETER_SENSOR)
 void flowrate_manage();
 void flow_init();
-int get_flowrate();
+float get_flowrate();
 #endif
 
 

MK/module/temperature/temperature.cpp

@@ -818,6 +818,10 @@ float get_pid_output(int h) {
 #if ENABLED(PIDTEMPCOOLER)
   float get_pid_output_cooler() {
      float pid_output;
+
+     if(target_temperature_cooler == 0)
+        return 0
+
      #if ENABLED(PID_OPENLOOP)
        pid_output = constrain(target_temperature_cooler, 0, MAX_COOLER_POWER);
      #else