diff --git a/DCCEXParser.cpp b/DCCEXParser.cpp
index 7610d45..49d2d20 100644
--- a/DCCEXParser.cpp
+++ b/DCCEXParser.cpp
@@ -638,12 +638,12 @@ void DCCEXParser::parseOne(Print *stream, byte *com, RingStream * ringStream)
     case ' ': // < >
         StringFormatter::send(stream, F("\n"));
         return;
-
+#ifndef DISABLE_DIAG
     case 'D': // < >
         if (parseD(stream, params, p))
             return;
         return;
-
+#endif
     case '=': // <= Track manager control  >
         if (TrackManager::parseJ(stream, params, p))
             return;
diff --git a/IODevice.h b/IODevice.h
index 769e111..4eb24e5 100644
--- a/IODevice.h
+++ b/IODevice.h
@@ -29,8 +29,13 @@
 
 // Define symbol IO_NO_HAL to reduce FLASH footprint when HAL features not required
 // The HAL is disabled by default on Nano and Uno platforms, because of limited flash space.
-#if defined(ARDUINO_AVR_NANO) || defined(ARDUINO_AVR_UNO) 
-#define IO_NO_HAL
+#include "defines.h"
+#if defined(ARDUINO_AVR_NANO) || defined(ARDUINO_AVR_UNO)
+  #if defined(DISABLE_DIAG) && defined(DISABLE_EEPROM) && defined(DISABLE_PROG)
+    #warning you have sacrificed DIAG for HAL
+  #else
+    #define IO_NO_HAL
+  #endif
 #endif
 
 // Define symbol IO_SWITCH_OFF_SERVO to set the PCA9685 output to 0 when an