✨ Temperature variance monitor (#23373)

Marlin/Configuration_adv.h

   #define WATCH_COOLER_TEMP_INCREASE            3 // Degrees Celsius
 #endif
 
+#if ANY(THERMAL_PROTECTION_HOTENDS, THERMAL_PROTECTION_BED, THERMAL_PROTECTION_CHAMBER, THERMAL_PROTECTION_COOLER)
+  #define THERMAL_PROTECTION_VARIANCE_MONITOR     // Detect a sensor malfunction preventing temperature updates
+#endif
+
 #if ENABLED(PIDTEMP)
   // Add an experimental additional term to the heater power, proportional to the extrusion speed.
   // A well-chosen Kc value should add just enough power to melt the increased material volume.

Marlin/src/core/language.h

 #define STR_REDUNDANCY                      "Heater switched off. Temperature difference between temp sensors is too high !"
 #define STR_T_HEATING_FAILED                "Heating failed"
 #define STR_T_THERMAL_RUNAWAY               "Thermal Runaway"
+#define STR_T_MALFUNCTION                   "Thermal Malfunction"
 #define STR_T_MAXTEMP                       "MAXTEMP triggered"
 #define STR_T_MINTEMP                       "MINTEMP triggered"
 #define STR_ERR_PROBING_FAILED              "Probing Failed"

Marlin/src/inc/Conditionals_post.h

 #if BOTH(HAS_COOLER, THERMAL_PROTECTION_COOLER) && WATCH_COOLER_TEMP_PERIOD > 0
   #define WATCH_COOLER 1
 #endif
+#if NONE(THERMAL_PROTECTION_HOTENDS, THERMAL_PROTECTION_CHAMBER, THERMAL_PROTECTION_BED, THERMAL_PROTECTION_COOLER)
+  #undef THERMAL_PROTECTION_VARIANCE_MONITOR
+#endif
 #if  (ENABLED(THERMAL_PROTECTION_HOTENDS) || !EXTRUDERS) \
   && (ENABLED(THERMAL_PROTECTION_BED)     || !HAS_HEATED_BED) \
   && (ENABLED(THERMAL_PROTECTION_CHAMBER) || !HAS_HEATED_CHAMBER) \

Marlin/src/lcd/language/language_el.h

   LSTR MSG_HEATING_FAILED_LCD             = _UxGT("Αποτυχία θέρμανσης");
   LSTR MSG_ERR_REDUNDANT_TEMP             = _UxGT("ΠΛΕΟΝΑΖΟΥΣΑ ΘΕΡΜΟΤΗΤΑ");
   LSTR MSG_THERMAL_RUNAWAY                = _UxGT("ΘΕΡΜΙΚΗ ΔΙΑΦΥΓΗ");
+  LSTR MSG_TEMP_MALFUNCTION               = _UxGT("ΘΕΡΜΙΚΗ ΔΥΣΛΕΙΤΟΥΡΓΙΑ");
   LSTR MSG_ERR_MAXTEMP                    = _UxGT("ΠΕΡΙΤΤΗ ΘΕΡΜΟΚΡΑΣΙΑ");
   LSTR MSG_ERR_MINTEMP                    = _UxGT("ΑΝΕΠΑΡΚΗΣ ΘΕΡΜΟΚΡΑΣΙΑ");
   LSTR MSG_HALTED                         = _UxGT("Εκτυπωτής διεκόπη");

Marlin/src/lcd/language/language_en.h

   LSTR MSG_HEATING_FAILED_LCD             = _UxGT("Heating Failed");
   LSTR MSG_ERR_REDUNDANT_TEMP             = _UxGT("Err: REDUNDANT TEMP");
   LSTR MSG_THERMAL_RUNAWAY                = _UxGT("THERMAL RUNAWAY");
+  LSTR MSG_TEMP_MALFUNCTION               = _UxGT("TEMP MALFUNCTION");
   LSTR MSG_THERMAL_RUNAWAY_BED            = _UxGT("BED THERMAL RUNAWAY");
   LSTR MSG_THERMAL_RUNAWAY_CHAMBER        = _UxGT("CHAMBER T. RUNAWAY");
   LSTR MSG_THERMAL_RUNAWAY_COOLER         = _UxGT("Cooler Runaway");

Marlin/src/module/temperature.cpp

 Temperature thermalManager;
 
 PGMSTR(str_t_thermal_runaway, STR_T_THERMAL_RUNAWAY);
+PGMSTR(str_t_temp_malfunction, STR_T_MALFUNCTION);
 PGMSTR(str_t_heating_failed, STR_T_HEATING_FAILED);
 
 /**
       );
     */
 
-    // If the heater idle timeout expires, restart
-    if (TERN0(HEATER_IDLE_HANDLER, heater_idle[idle_index].timed_out)) {
-      state = TRInactive;
-      running_temp = 0;
-    }
-    else if (running_temp != target) { // If the target temperature changes, restart
-      running_temp = target;
-      state = target > 0 ? TRFirstHeating : TRInactive;
+    #if ENABLED(THERMAL_PROTECTION_VARIANCE_MONITOR)
+      if (state == TRMalfunction) { // temperature invariance may continue, regardless of heater state
+        variance += ABS(current - last_temp); // no need for detection window now, a single change in variance is enough
+        last_temp = current;
+        if (!NEAR_ZERO(variance)) {
+          variance_timer = millis() + SEC_TO_MS(period_seconds);
+          variance = 0.0;
+          state = TRStable; // resume from where we detected the problem
+        }
+      }
+    #endif
+
+    if (TERN1(THERMAL_PROTECTION_VARIANCE_MONITOR, state != TRMalfunction)) {
+      // If the heater idle timeout expires, restart
+      if (TERN0(HEATER_IDLE_HANDLER, heater_idle[idle_index].timed_out)) {
+        state = TRInactive;
+        running_temp = 0;
+        TERN_(THERMAL_PROTECTION_VARIANCE_MONITOR, variance_timer = 0);
+      }
+      else if (running_temp != target) { // If the target temperature changes, restart
+        running_temp = target;
+        state = target > 0 ? TRFirstHeating : TRInactive;
+        TERN_(THERMAL_PROTECTION_VARIANCE_MONITOR, variance_timer = 0);
+      }
     }
 
     switch (state) {
 
         const millis_t now = millis();
 
+        #if ENABLED(THERMAL_PROTECTION_VARIANCE_MONITOR)
+          if (PENDING(now, variance_timer)) {
+            variance += ABS(current - last_temp);
+            last_temp = current;
+          }
+          else {
+            if (NEAR_ZERO(variance) && variance_timer) { // valid variance monitoring window
+              state = TRMalfunction;
+              break;
+            }
+            variance_timer = now + SEC_TO_MS(period_seconds);
+            variance = 0.0;
+            last_temp = current;
+          }
+        #endif
+
         if (current >= running_temp - hysteresis_degc) {
           timer = now + SEC_TO_MS(period_seconds);
           break;
       case TRRunaway:
         TERN_(HAS_DWIN_E3V2_BASIC, DWIN_Popup_Temperature(0));
         _temp_error(heater_id, FPSTR(str_t_thermal_runaway), GET_TEXT_F(MSG_THERMAL_RUNAWAY));
+
+      #if ENABLED(THERMAL_PROTECTION_VARIANCE_MONITOR)
+        case TRMalfunction:
+          TERN_(HAS_DWIN_E3V2_BASIC, DWIN_Popup_Temperature(0));
+          _temp_error(heater_id, FPSTR(str_t_temp_malfunction), GET_TEXT_F(MSG_TEMP_MALFUNCTION));
+      #endif
     }
   }
 

Marlin/src/module/temperature.h

         return (RunawayIndex)_MAX(heater_id, 0);
       }
 
-      enum TRState : char { TRInactive, TRFirstHeating, TRStable, TRRunaway };
+      enum TRState : char { TRInactive, TRFirstHeating, TRStable, TRRunaway
+        OPTARG(THERMAL_PROTECTION_VARIANCE_MONITOR, TRMalfunction)
+      };
 
       typedef struct {
         millis_t timer = 0;
         TRState state = TRInactive;
         float running_temp;
+        #if ENABLED(THERMAL_PROTECTION_VARIANCE_MONITOR)
+          millis_t variance_timer = 0;
+          celsius_float_t last_temp = 0.0, variance = 0.0;
+        #endif
         void run(const_celsius_float_t current, const_celsius_float_t target, const heater_id_t heater_id, const uint16_t period_seconds, const celsius_t hysteresis_degc);
       } tr_state_machine_t;