diff --git a/Marlin/src/HAL/HAL_LPC1768/HAL.h b/Marlin/src/HAL/HAL_LPC1768/HAL.h
index 24dea7530..5a11dfd67 100644
--- a/Marlin/src/HAL/HAL_LPC1768/HAL.h
+++ b/Marlin/src/HAL/HAL_LPC1768/HAL.h
@@ -147,8 +147,8 @@ using FilteredADC = LPC176x::ADC<ADC_LOWPASS_K_VALUE, ADC_MEDIAN_FILTER_SIZE>;
 #define HAL_READ_ADC()         FilteredADC::get_result()
 #define HAL_ADC_READY()        FilteredADC::finished_conversion()
 
-// A grace period for the ADC readings to stabilize before they start causing thermal protection errors.
-#define THERMAL_PROTECTION_GRACE_PERIOD 800
+// A grace period to allow ADC readings to stabilize, preventing false alarms
+#define THERMAL_PROTECTION_GRACE_PERIOD 1000
 
 // Parse a G-code word into a pin index
 int16_t PARSED_PIN_INDEX(const char code, const int16_t dval);
diff --git a/Marlin/src/module/temperature.cpp b/Marlin/src/module/temperature.cpp
index 25b1f5990..16d551a9a 100644
--- a/Marlin/src/module/temperature.cpp
+++ b/Marlin/src/module/temperature.cpp
@@ -1531,7 +1531,7 @@ void Temperature::init() {
     }while(0)
     #define _TEMP_MAX_E(NR) do{ \
       temp_range[NR].maxtemp = HEATER_ ##NR## _MAXTEMP; \
-      while (analog_to_celsius_hotend(temp_range[NR].raw_min, NR) > HEATER_ ##NR## _MAXTEMP) \
+      while (analog_to_celsius_hotend(temp_range[NR].raw_max, NR) > HEATER_ ##NR## _MAXTEMP) \
         temp_range[NR].raw_max -= TEMPDIR(NR) * (OVERSAMPLENR); \
     }while(0)
 
@@ -1589,7 +1589,7 @@ void Temperature::init() {
       while (analog_to_celsius_bed(mintemp_raw_BED) < BED_MINTEMP) mintemp_raw_BED += TEMPDIR(BED) * (OVERSAMPLENR);
     #endif
     #ifdef BED_MAXTEMP
-      while (analog_to_celsius_bed(maxtemp_raw_BED) > BED_MAXTEMP) mintemp_raw_BED -= TEMPDIR(BED) * (OVERSAMPLENR);
+      while (analog_to_celsius_bed(maxtemp_raw_BED) > BED_MAXTEMP) maxtemp_raw_BED -= TEMPDIR(BED) * (OVERSAMPLENR);
     #endif
   #endif // HAS_HEATED_BED
 
@@ -1598,7 +1598,7 @@ void Temperature::init() {
       while (analog_to_celsius_chamber(mintemp_raw_CHAMBER) < CHAMBER_MINTEMP) mintemp_raw_CHAMBER += TEMPDIR(CHAMBER) * (OVERSAMPLENR);
     #endif
     #ifdef CHAMBER_MAXTEMP
-      while (analog_to_celsius_chamber(maxtemp_raw_CHAMBER) > CHAMBER_MAXTEMP) mintemp_raw_CHAMBER -= TEMPDIR(CHAMBER) * (OVERSAMPLENR);
+      while (analog_to_celsius_chamber(maxtemp_raw_CHAMBER) > CHAMBER_MAXTEMP) maxtemp_raw_CHAMBER -= TEMPDIR(CHAMBER) * (OVERSAMPLENR);
     #endif
   #endif