Fix mintemp/maxtemp monitoring for thermocouples

Marlin/src/module/temperature.cpp

@@ -2194,14 +2194,18 @@ void Temperature::readings_ready() {
     temp_chamber.acc = 0;
   #endif
 
-  int constexpr temp_dir[] = {
+  static constexpr int8_t temp_dir[] = {
     #if ENABLED(HEATER_0_USES_MAX6675)
       0
     #else
       TEMPDIR(0)
     #endif
     #if HOTENDS > 1
+      #if ENABLED(HEATER_1_USES_MAX6675)
+        , 0
+      #else
         , TEMPDIR(1)
+      #endif
       #if HOTENDS > 2
         , TEMPDIR(2)
         #if HOTENDS > 3
@@ -2221,12 +2225,14 @@ void Temperature::readings_ready() {
   if (grace_period) return;
 
   for (uint8_t e = 0; e < COUNT(temp_dir); e++) {
-    const int16_t tdir = temp_dir[e], rawtemp = temp_hotend[e].raw * tdir;
+    const int8_t tdir = temp_dir[e];
-    const bool heater_on = (temp_hotend[e].target > 0)
+    if (tdir) {
+      const int16_t rawtemp = temp_hotend[e].raw * tdir; // normal direction, +rawtemp, else -rawtemp
+      const bool heater_on = (temp_hotend[e].target > 0
         #if ENABLED(PIDTEMP)
-        || (temp_hotend[e].soft_pwm_amount > 0)
+          || temp_hotend[e].soft_pwm_amount > 0
         #endif
-    ;
+      );
       if (rawtemp > temp_range[e].raw_max * tdir) max_temp_error(e);
       if (heater_on && rawtemp < temp_range[e].raw_min * tdir && !is_preheating(e)) {
         #ifdef MAX_CONSECUTIVE_LOW_TEMPERATURE_ERROR_ALLOWED
@@ -2239,6 +2245,7 @@ void Temperature::readings_ready() {
           consecutive_low_temperature_error[e] = 0;
       #endif
     }
+  }
 
   #if HAS_HEATED_BED
     #if TEMPDIR(BED) < 0