diff --git a/modules/libcom/src/error/Makefile b/modules/libcom/src/error/Makefile
index 570974b95..f98ed4b24 100644
--- a/modules/libcom/src/error/Makefile
+++ b/modules/libcom/src/error/Makefile
@@ -26,6 +26,7 @@ ERR_S_FILES += $(LIBCOM)/as/asLib.h
 ERR_S_FILES += $(LIBCOM)/misc/epicsStdlib.h
 ERR_S_FILES += $(LIBCOM)/pool/epicsThreadPool.h
 ERR_S_FILES += $(LIBCOM)/error/errMdef.h
+ERR_S_FILES += $(LIBCOM)/error/errSymTbl.h
 ERR_S_FILES += $(TOP)/modules/database/src/ioc/db/dbAccessDefs.h
 ERR_S_FILES += $(TOP)/modules/database/src/ioc/dbStatic/dbStaticLib.h
 
diff --git a/modules/libcom/src/error/errMdef.h b/modules/libcom/src/error/errMdef.h
index 1a9c47e33..afda69ca9 100644
--- a/modules/libcom/src/error/errMdef.h
+++ b/modules/libcom/src/error/errMdef.h
@@ -32,6 +32,7 @@
 #define M_stdlib        (504 << 16) /* EPICS Standard library */
 #define M_pool          (505 << 16) /* Thread pool */
 #define M_time          (506 << 16) /* epicsTime */
+#define M_err           (507 << 16) /* Error */
 
 /* ioc */
 #define M_dbAccess      (511 << 16) /* Database Access Routines */
diff --git a/modules/libcom/src/error/errSymLib.c b/modules/libcom/src/error/errSymLib.c
index 14fa21904..deb59d4f8 100644
--- a/modules/libcom/src/error/errSymLib.c
+++ b/modules/libcom/src/error/errSymLib.c
@@ -33,14 +33,12 @@
 static epicsUInt16 errhash(long errNum);
 
 typedef struct errnumnode {
-    ELLNODE            node;
     long               errNum;
     struct errnumnode *hashnode;
     const char        *message;
     long               pad;
 } ERRNUMNODE;
 
-static ELLLIST errnumlist = ELLLIST_INIT;
 static ERRNUMNODE **hashtable;
 static int initialized = 0;
 extern ERRSYMTAB_ID errSymTbl;
@@ -56,44 +54,20 @@ extern ERRSYMTAB_ID errSymTbl;
 int errSymBld(void)
 {
     ERRSYMBOL      *errArray = errSymTbl->symbols;
-    ERRNUMNODE     *perrNumNode = NULL;
-    ERRNUMNODE     *pNextNode = NULL;
-    ERRNUMNODE    **phashnode = NULL;
     int             i;
-    int             modnum;
 
     if (initialized)
         return(0);
 
     hashtable = (ERRNUMNODE**)callocMustSucceed
         (NHASH, sizeof(ERRNUMNODE*),"errSymBld");
-    for (i = 0; i < errSymTbl->nsymbols; i++, errArray++) {
-        modnum = errArray->errNum >> 16;
-        if (modnum < 501) {
-            fprintf(stderr, "errSymBld: ERROR - Module number in errSymTbl < 501 was Module=%lx Name=%s\n",
-                errArray->errNum, errArray->name);
-            continue;
-        }
-        if ((errSymbolAdd(errArray->errNum, errArray->name)) < 0) {
-            fprintf(stderr, "errSymBld: ERROR - errSymbolAdd() failed \n");
-            continue;
-        }
-    }
-    perrNumNode = (ERRNUMNODE *) ellFirst(&errnumlist);
-    while (perrNumNode) {
-        /* hash each perrNumNode->errNum */
-        epicsUInt16 hashInd = errhash(perrNumNode->errNum);
 
-        phashnode = (ERRNUMNODE**)&hashtable[hashInd];
-        pNextNode = (ERRNUMNODE*) *phashnode;
-        /* search for last node (NULL) of hashnode linked list */
-        while (pNextNode) {
-            phashnode = &pNextNode->hashnode;
-            pNextNode = *phashnode;
+    for (i = 0; i < errSymTbl->nsymbols; i++, errArray++) {
+        if (errSymbolAdd(errArray->errNum, errArray->name)) {
+            fprintf(stderr, "errSymBld: ERROR - errSymbolAdd() failed \n");
         }
-        *phashnode = perrNumNode;
-        perrNumNode = (ERRNUMNODE *) ellNext((ELLNODE *) perrNumNode);
     }
+
     initialized = 1;
     return(0);
 }
@@ -114,16 +88,38 @@ static epicsUInt16 errhash(long errNum)
 
 /****************************************************************
  * ERRSYMBOLADD
- * adds symbols to the master errnumlist as compiled from errSymTbl.c
+ * adds symbols to the global error symbol hash table
  ***************************************************************/
 int errSymbolAdd(long errNum, const char *name)
 {
-    ERRNUMNODE *pNew = (ERRNUMNODE*) callocMustSucceed(1,
-        sizeof(ERRNUMNODE), "errSymbolAdd");
+    ERRNUMNODE *pNextNode = NULL;
+    ERRNUMNODE **phashnode = NULL;
+    ERRNUMNODE *pNew = NULL;
+    int modnum = (epicsUInt16) (errNum >> 16);
 
+    if (modnum < 501)
+        return S_err_invCode;
+
+    epicsUInt16 hashInd = errhash(errNum);
+
+    phashnode = (ERRNUMNODE**)&hashtable[hashInd];
+    pNextNode = (ERRNUMNODE*) *phashnode;
+
+    /* search for last node (NULL) of hashnode linked list */
+    while (pNextNode) {
+        if (pNextNode->errNum == errNum) {
+            return S_err_codeExists;
+        }
+        phashnode = &pNextNode->hashnode;
+        pNextNode = *phashnode;
+    }
+
+    pNew = (ERRNUMNODE*) callocMustSucceed(
+        1, sizeof(ERRNUMNODE), "errSymbolAdd");
     pNew->errNum = errNum;
     pNew->message = name;
-    ellAdd(&errnumlist, (ELLNODE*)pNew);
+    *phashnode = pNew;
+
     return 0;
 }
 
diff --git a/modules/libcom/src/error/errSymTbl.h b/modules/libcom/src/error/errSymTbl.h
index 66dcf9de8..3c009ee60 100644
--- a/modules/libcom/src/error/errSymTbl.h
+++ b/modules/libcom/src/error/errSymTbl.h
@@ -16,6 +16,9 @@
 #include "libComAPI.h"
 #include "epicsTypes.h"
 
+#define S_err_invCode (M_err | 1)       /* Invalid error symbol code */
+#define S_err_codeExists (M_err | 2)    /* Error code already exists */
+
 /* ERRSYMBOL - entry in symbol table */
 typedef struct {
     long errNum;        /* errMessage symbol number */
diff --git a/modules/libcom/test/epicsErrlogTest.c b/modules/libcom/test/epicsErrlogTest.c
index 16a04a73c..59a149cb5 100644
--- a/modules/libcom/test/epicsErrlogTest.c
+++ b/modules/libcom/test/epicsErrlogTest.c
@@ -29,6 +29,7 @@
 #include "osiSock.h"
 #include "fdmgr.h"
 #include "epicsString.h"
+#include "errSymTbl.h"
 
 /* private between errlog.c and this test */
 LIBCOM_API
@@ -197,13 +198,53 @@ void testANSIStrip(void)
 #undef testEscape
 }
 
+static void testErrorMessageMatches(long status, const char *expected)
+{
+    const char *msg = errSymMsg(status);
+    testOk(strcmp(msg, expected) == 0,
+           "Error code %ld returns \"%s\", expected message \"%s\"", status,
+           msg, expected
+          );
+}
+
+static void testGettingExistingErrorSymbol()
+{
+    testErrorMessageMatches(S_err_invCode, "Invalid error symbol code");
+}
+
+static void testGettingNonExistingErrorSymbol()
+{
+    long invented_code = (0x7999 << 16) | 0x9998;
+    testErrorMessageMatches(invented_code, "<Unknown code>");
+}
+
+static void testAddingErrorSymbol()
+{
+    long invented_code = (0x7999 << 16) | 0x9999;
+    errSymbolAdd(invented_code, "Invented Error Message");
+    testErrorMessageMatches(invented_code, "Invented Error Message");
+}
+
+static void testAddingInvalidErrorSymbol()
+{
+    long invented_code = (500 << 16) | 0x1;
+    testOk(errSymbolAdd(invented_code, "No matter"),
+           "Adding error symbol with module code < 501 should fail");
+}
+
+static void testAddingExistingErrorSymbol()
+{
+    testOk(errSymbolAdd(S_err_invCode, "Duplicate"),
+           "Adding an error symbol with an existing error code should fail");
+}
+
 MAIN(epicsErrlogTest)
 {
     size_t mlen, i, N;
     char msg[256];
     clientPvt pvt, pvt2;
 
-    testPlan(48);
+    testPlan(53);
 
     testANSIStrip();
 
@@ -414,6 +455,12 @@ MAIN(epicsErrlogTest)
     testLogPrefix();
     osiSockRelease();
 
+    testGettingExistingErrorSymbol();
+    testGettingNonExistingErrorSymbol();
+    testAddingErrorSymbol();
+    testAddingInvalidErrorSymbol();
+    testAddingExistingErrorSymbol();
+
     return testDone();
 }
 /*