Index: firmware/App/Services/SystemComm.c
===================================================================
diff -u -r6703d916afad5bfc0f5282e8ff6a09d8ce6d9180 -r7d4711edd7b40cd3e29f43e766f79a8a09586fe9
--- firmware/App/Services/SystemComm.c	(.../SystemComm.c)	(revision 6703d916afad5bfc0f5282e8ff6a09d8ce6d9180)
+++ firmware/App/Services/SystemComm.c	(.../SystemComm.c)	(revision 7d4711edd7b40cd3e29f43e766f79a8a09586fe9)
@@ -1,14 +1,14 @@
 /**************************************************************************
 *
-* Copyright (c) 2019-2023 Diality Inc. - All Rights Reserved.
+* Copyright (c) 2019-2024 Diality Inc. - All Rights Reserved.
 *
 * THIS CODE MAY NOT BE COPIED OR REPRODUCED IN ANY FORM, IN PART OR IN
 * WHOLE, WITHOUT THE EXPLICIT PERMISSION OF THE COPYRIGHT OWNER.
 *
 * @file    SystemComm.c
 *
 * @author  (last)      Sean Nash
-* @date    (last)      24-Aug-2023
+* @date    (last)      05-Oct-2023
 *
 * @author  (original)  Dara Navaei
 * @date    (original)  05-Nov-2019
@@ -21,14 +21,15 @@
 #include "sci.h"
 #include "sys_dma.h"
 
-#include "DrainPump.h"
-#include "SystemComm.h"
 #include "Comm.h"
+#include "DrainPump.h"
+#include "Heaters.h"
 #include "Interrupts.h"
 #include "OperationModes.h"
+#include "SystemComm.h"
+#include "SystemCommMessages.h"
 #include "Timers.h"
 #include "Utilities.h"
-#include "SystemCommMessages.h"
 
 /**
  *  @addtogroup SystemComm
@@ -56,6 +57,9 @@
 #define MAX_FPGA_CLOCK_SPEED_ERRORS             3                           ///< maximum number of FPGA clock speed errors within window period before alarm
 #define MAX_FPGA_CLOCK_SPEED_ERROR_WINDOW_MS    (10 * SEC_PER_MIN * MS_PER_SECOND)    ///< FPGA clock speed error window
 
+#define PENDING_ACK_LIST_OVERRIDE_UI_CHANNEL    1                                     ///< Value for determining UI channel when Pending ACKs are overriden.
+#define PENDING_ACK_LIST_OVERRIDE_DG_CHANNEL    2                                     ///< Value for determining HD channel when Pending ACKs are overriden.
+
 #pragma pack(push, 1)
 
 /// Record for transmitted message that is pending acknowledgement from receiver.
@@ -669,7 +673,8 @@
     if ( TRUE == didTimeout( timeOfLastHDCheckIn, HD_COMM_TIMEOUT_IN_MS ) )
     {
         hdCommunicationStatus.data = FALSE;
-        setHDOperationMode( 0, 0 );   // If HD off or not connected, consider HD mode is fault.
+        setHDOperationMode( 0, 0 );      // If HD off or not connected, consider HD mode is fault.
+        stopHeater( DG_TRIMMER_HEATER ); // If HD off or not connected, ensure trimmer heater is off.
     }
 }
 
@@ -750,13 +755,18 @@
     BOOL result = FALSE;
     U32  i;
 
-    // find match
+    // Find match
     for ( i = 0; i < PENDING_ACK_LIST_SIZE; i++ )
     {
         if ( ( TRUE == pendingAckList[ i ].used ) && ( pendingAckList[ i ].seqNo == seqNo ) )
-        {   // remove message pending ACK from list
-            pendingAckList[ i ].used = FALSE;
+        {
             result = TRUE;
+            // Remove message pending ACK from list
+            if ( ( ( getU32OverrideValue( &pendingACKOverride ) != PENDING_ACK_LIST_OVERRIDE_UI_CHANNEL ) || ( pendingAckList[ i ].channel != COMM_BUFFER_OUT_CAN_DG_2_UI ) ) &&
+                 ( ( getU32OverrideValue( &pendingACKOverride ) != PENDING_ACK_LIST_OVERRIDE_DG_CHANNEL ) || ( pendingAckList[ i ].channel != COMM_BUFFER_OUT_CAN_DG_2_HD ) ) )
+            {
+                pendingAckList[ i ].used = FALSE;
+            }
             break;
         }
     }
@@ -782,7 +792,7 @@
     {
         for ( i = 0; i < PENDING_ACK_LIST_SIZE; i++ )
         {   // pending ACK expired?
-            if ( ( TRUE == pendingAckList[ i ].used ) && ( ( TRUE == didTimeout( pendingAckList[ i ].timeStamp, MSG_NOT_ACKED_TIMEOUT_MS ) ) ||  ( getU32OverrideValue( &pendingACKOverride ) > 0 ) ) )
+            if ( ( TRUE == pendingAckList[ i ].used ) && ( TRUE == didTimeout( pendingAckList[ i ].timeStamp, MSG_NOT_ACKED_TIMEOUT_MS ) ) )
             {   // if retries left, reset and resend pending message. Do not retry when in POST since the UI might not still be responsive
                 if ( pendingAckList[ i ].retries > 0 )
                 {
@@ -883,6 +893,10 @@
             handleHDStartStopTrimmerHeaterCmd( message );
             break;
 
+        case MSG_ID_DG_PARK_CONCENTRATE_PUMPS_CMD_REQUEST:
+            handleParkConecentratePumpsCmd( message );
+            break;
+
         case MSG_ID_DG_START_STOP_TRIMMER_HEATER_CMD:
             handleDGStartStopTrimmerHeater( message );
             break;
@@ -1386,6 +1400,10 @@
                 handleTestDG77CStateTimerOverride( message );
                 break;
 
+            case MSG_ID_DG_CHEM_DISINFECT_ACID_OVERRIDE:
+                handleTestChemDisinfectAcidOverride( message );
+                break;
+
             default:
                 // TODO - unrecognized message ID received - ignore
                 break;