Add option WAIT_FOR_HOTEND under PROBING_HEATERS_OFF.

Marlin/Configuration.h

@@ -1139,6 +1139,7 @@
 //#define PROBING_HEATERS_OFF       // Turn heaters off when probing
 #if ENABLED(PROBING_HEATERS_OFF)
   //#define WAIT_FOR_BED_HEATER     // Wait for bed to heat back up between probes (to improve accuracy)
+  //#define WAIT_FOR_HOTEND         // Wait for hotend to heat back up between probes (to improve accuracy & prevent undertemp extrudes)
 #endif
 //#define PROBING_FANS_OFF          // Turn fans off when probing
 //#define PROBING_STEPPERS_OFF      // Turn steppers off (unless needed to hold position) when probing

Marlin/src/module/motion.cpp

@@ -1318,11 +1318,16 @@ void prepare_line_to_destination() {
     if (is_home_dir) {
 
       if (TERN0(HOMING_Z_WITH_PROBE, axis == Z_AXIS)) {
-        #if ALL(HAS_HEATED_BED, WAIT_FOR_BED_HEATER)
+        #if BOTH(HAS_HEATED_BED, WAIT_FOR_BED_HEATER)
           // Wait for bed to heat back up between probing points
           thermalManager.wait_for_bed_heating();
         #endif
 
+        #if BOTH(HAS_HOTEND, WAIT_FOR_HOTEND)
+          // Wait for the hotend to heat back up between probing points
+          thermalManager.wait_for_hotend_heating(active_extruder);
+        #endif
+
         TERN_(HAS_QUIET_PROBING, if (final_approach) probe.set_probing_paused(true));
       }
 

Marlin/src/module/probe.cpp

@@ -478,6 +478,10 @@ bool Probe::probe_down_to_z(const float z, const feedRate_t fr_mm_s) {
     thermalManager.wait_for_bed_heating();
   #endif
 
+  #if BOTH(HAS_HOTEND, WAIT_FOR_HOTEND)
+    thermalManager.wait_for_hotend_heating(active_extruder);
+  #endif
+
   if (TERN0(BLTOUCH_SLOW_MODE, bltouch.deploy())) return true; // Deploy in LOW SPEED MODE on every probe action
 
   // Disable stealthChop if used. Enable diag1 pin on driver.

Marlin/src/module/temperature.cpp

@@ -3447,6 +3447,15 @@ void Temperature::tick() {
       return false;
     }
 
+    void Temperature::wait_for_hotend_heating(const uint8_t target_extruder) {
+      if (isHeatingHotend(target_extruder)) {
+        SERIAL_ECHOLNPGM("Wait for hotend heating...");
+        LCD_MESSAGEPGM(MSG_HEATING);
+        wait_for_hotend(target_extruder);
+        ui.reset_status();
+      }
+    }
+
   #endif // HAS_TEMP_HOTEND
 
   #if HAS_HEATED_BED

Marlin/src/module/temperature.h

@@ -630,6 +630,8 @@ class Temperature {
             , const bool click_to_cancel=false
           #endif
         );
+
+        static void wait_for_hotend_heating(const uint8_t target_extruder);
       #endif
 
       FORCE_INLINE static bool still_heating(const uint8_t e) {