Index: firmware/App/Services/SystemComm.c
===================================================================
diff -u -r6c60d0d0300828750c76f2e397d4e7ccaa84bbb2 -r05b4d416b6bd5e7af4347f112a20f9f81a257064
--- firmware/App/Services/SystemComm.c	(.../SystemComm.c)	(revision 6c60d0d0300828750c76f2e397d4e7ccaa84bbb2)
+++ firmware/App/Services/SystemComm.c	(.../SystemComm.c)	(revision 05b4d416b6bd5e7af4347f112a20f9f81a257064)
@@ -844,16 +844,17 @@
  *************************************************************************/
 static void checkTooManyBadMsgCRCs( U16 msgID )
 {
-    U32 listTimeInMS = calcTimeSince( badCRCTimeStamps[ badCRCListIdx ] );
+    U32 listTimeInMS;
 
     // replace oldest bad CRC in list with this new one
     badCRCTimeStamps[ badCRCListIdx ] = getMSTimerCount();
     // move list index to next position (may wrap)
-    badCRCListIdx = INC_WRAP( badCRCListIdx, 0, MAX_COMM_CRC_FAILURES );
+    badCRCListIdx = INC_WRAP( badCRCListIdx, 0, MAX_COMM_CRC_FAILURES - 1 );
     // update list count
     badCRCListCount = INC_CAP( badCRCListCount, MAX_COMM_CRC_FAILURES );
     // check if too many bad CRCs in window of time
-    if ( listTimeInMS >= MAX_COMM_CRC_FAILURE_WINDOW_MS )
+    listTimeInMS = calcTimeSince( badCRCTimeStamps[ badCRCListIdx ] );
+    if ( listTimeInMS <= MAX_COMM_CRC_FAILURE_WINDOW_MS )
     {
         SET_ALARM_WITH_1_U32_DATA( ALARM_ID_COMM_TOO_MANY_BAD_CRCS, (U32)msgID );
     }
@@ -884,14 +885,10 @@
         _disable_IRQ();  // slot selection needs interrupt protection
         if ( FALSE == pendingAckList[ i ].used )
         {
-            S16 seqNo = msg->hdr.seqNo;
+            S16 seqNo = msg->hdr.seqNo * -1;  // remove ACK bit from seq #
 
             pendingAckList[ i ].used = TRUE;
             _enable_IRQ();
-            if ( seqNo < 0 )
-            {
-                seqNo *= -1;
-            }
             pendingAckList[ i ].seqNo = seqNo;
             pendingAckList[ i ].channel = channel;
             pendingAckList[ i ].timeStamp = getMSTimerCount();
Index: firmware/App/Services/SystemCommMessages.c
===================================================================
diff -u -r6c60d0d0300828750c76f2e397d4e7ccaa84bbb2 -r05b4d416b6bd5e7af4347f112a20f9f81a257064
--- firmware/App/Services/SystemCommMessages.c	(.../SystemCommMessages.c)	(revision 6c60d0d0300828750c76f2e397d4e7ccaa84bbb2)
+++ firmware/App/Services/SystemCommMessages.c	(.../SystemCommMessages.c)	(revision 05b4d416b6bd5e7af4347f112a20f9f81a257064)
@@ -109,16 +109,19 @@
     // prefix data with message sync byte
     data[ msgSize++ ] = MESSAGE_SYNC_BYTE;
 
-    // set sequence # and ACK bit
+    // set sequence # and ACK bit (unless this is an ACK to a received message)
 #ifndef ACK_NOT_IMPLEMENTED
-    // thread protect next sequence # access & increment
-    _disable_IRQ();
-        msg.hdr.seqNo = nextSeqNo;
-        nextSeqNo = INC_WRAP( nextSeqNo, MIN_MSG_SEQ_NO, MAX_MSG_SEQ_NO );
-    _enable_IRQ();
-    if ( TRUE == ackReq )
+    if ( msg.hdr.msgID != MSG_ID_ACK )
     {
-        msg.hdr.seqNo *= -1;
+        // thread protect next sequence # access & increment
+        _disable_IRQ();
+            msg.hdr.seqNo = nextSeqNo;
+            nextSeqNo = INC_WRAP( nextSeqNo, MIN_MSG_SEQ_NO, MAX_MSG_SEQ_NO );
+        _enable_IRQ();
+        if ( TRUE == ackReq )
+        {
+            msg.hdr.seqNo *= -1;
+        }
     }
 #endif