Output error for disabled M303

Marlin/Marlin_main.cpp

@@ -5559,6 +5559,7 @@ inline void gcode_M226() {
  *       U<bool> with a non-zero value will apply the result to current settings
  */
 inline void gcode_M303() {
+#if ENABLED(PIDTEMP)
   int e = code_seen('E') ? code_value_short() : 0;
   int c = code_seen('C') ? code_value_short() : 5;
   bool u = code_seen('U') && code_value_short() != 0;
@@ -5573,6 +5574,10 @@ inline void gcode_M303() {
   PID_autotune(temp, e, c, u);
 
   KEEPALIVE_STATE(IN_HANDLER);
+#else
+  SERIAL_ERROR_START;
+  SERIAL_ERRORLNPGM(MSG_ERR_M303_DISABLED);
+#endif
 }
 
 #if ENABLED(SCARA)

Marlin/language.h

@@ -159,6 +159,7 @@
 #define MSG_ERR_M421_REQUIRES_XYZ           "M421 requires XYZ parameters"
 #define MSG_ERR_MESH_INDEX_OOB              "Mesh XY index is out of bounds"
 #define MSG_ERR_M428_TOO_FAR                "Too far from reference point"
+#define MSG_ERR_M303_DISABLED               "PIDTEMP disabled"
 #define MSG_M119_REPORT                     "Reporting endstop status"
 #define MSG_ENDSTOP_HIT                     "TRIGGERED"
 #define MSG_ENDSTOP_OPEN                    "open"

Marlin/temperature.cpp

@@ -221,6 +221,8 @@ static void updateTemperaturesFromRawValues();
 //================================ Functions ================================
 //===========================================================================
 
+#if ENABLED(PIDTEMP)
+
 void PID_autotune(float temp, int extruder, int ncycles, bool set_result/*=false*/) {
   float input = 0.0;
   int cycles = 0;
@@ -392,6 +394,8 @@ void PID_autotune(float temp, int extruder, int ncycles, bool set_result/*=false
   }
 }
 
+#endif // PIDTEMP
+
 void updatePID() {
   #if ENABLED(PIDTEMP)
     for (int e = 0; e < EXTRUDERS; e++) {

Marlin/temperature.h

@@ -169,7 +169,9 @@ int getHeaterPower(int heater);
 void disable_all_heaters();
 void updatePID();
 
-void PID_autotune(float temp, int extruder, int ncycles, bool set_result=false);
+#if ENABLED(PIDTEMP)
+  void PID_autotune(float temp, int extruder, int ncycles, bool set_result=false);
+#endif
 
 void setExtruderAutoFanState(int pin, bool state);
 void checkExtruderAutoFans();