diff --git a/mode_s.c b/mode_s.c
index 3e7d058..14e895e 100644
--- a/mode_s.c
+++ b/mode_s.c
@@ -259,12 +259,9 @@ static int correct_aa_field(uint32_t *addr, struct errorinfo *ei)
 //  350: DF17/18 with 2-bit error and an address not matching a known aircraft
 
 // 1600: DF11 with IID==0, good CRC and an address matching a known aircraft
-//  800: DF11 with IID==0, 1-bit error and an address matching a known aircraft
-//  750: DF11 with IID==0, good CRC and an address not matching a known aircraft
-//  375: DF11 with IID==0, 1-bit error and an address not matching a known aircraft
-
 // 1000: DF11 with IID!=0, good CRC and an address matching a known aircraft
-//  500: DF11 with IID!=0, 1-bit error and an address matching a known aircraft
+//  800: DF11 with 1-bit error and an address matching a known aircraft
+//  750: DF11 with IID==0, good CRC and an address not matching a known aircraft
 
 // 1000: DF20/21 with a CRC-derived address matching a known aircraft
 //  500: DF20/21 with a CRC-derived address matching a known aircraft (bottom 16 bits only - overlay control in use)
@@ -310,31 +307,40 @@ int scoreModesMessage(unsigned char *msg, int validbits)
 
     case 11: // All-call reply
         iid = crc & 0x7f;
-        crc = crc & 0xffff80;
         addr = getbits(msg, 9, 32);
 
-        ei = modesChecksumDiagnose(crc, msgbits);
-        if (!ei)
-            return -2; // can't correct errors
+        if (crc & 0xffff80) {
+            // Try to diagnose based on the _full_ CRC
+            // i.e. under the assumption that IID = 0
+            ei = modesChecksumDiagnose(crc, msgbits);
+            if (!ei)
+                return -2; // can't correct errors
 
-        // see crc.c comments: we do not attempt to fix
-        // more than single-bit errors, as two-bit
-        // errors are ambiguous in DF11.
-        if (ei->errors > 1)
-            return -2; // can't correct errors
+            // see crc.c comments: we do not attempt to fix
+            // more than single-bit errors, as two-bit
+            // errors are ambiguous in DF11.
+            if (ei->errors > 1)
+                return -2; // can't correct errors
 
-        // fix any errors in the address field
-        correct_aa_field(&addr, ei);
+            // fix any errors in the address field
+            correct_aa_field(&addr, ei);
 
-        // validate address
+            // here, IID = 0 implicitly
+            if (icaoFilterTest(addr))
+                return 800;
+            else
+                return -1;
+        }
+
+        // CRC was correct (ish)
         if (iid == 0) {
             if (icaoFilterTest(addr))
-                return 1600 / (ei->errors + 1);
+                return 1600;
             else
-                return 750 / (ei->errors + 1);
-        } else {
+                return 750;
+        } else { // iid != 0
             if (icaoFilterTest(addr))
-                return 1000 / (ei->errors + 1);
+                return 1000;
             else
                 return -1;
         }
@@ -439,8 +445,11 @@ int decodeModesMessage(struct modesMessage *mm, unsigned char *msg)
 
         mm->IID = mm->crc & 0x7f;
         if (mm->crc & 0xffff80) {
+            // Try to diagnose based on the _full_ CRC
+            // i.e. under the assumption that IID = 0
+
             int addr;
-            struct errorinfo *ei = modesChecksumDiagnose(mm->crc & 0xffff80, mm->msgbits);
+            struct errorinfo *ei = modesChecksumDiagnose(mm->crc, mm->msgbits);
             if (!ei) {
                 return -2; // couldn't fix it
             }
@@ -452,6 +461,7 @@ int decodeModesMessage(struct modesMessage *mm, unsigned char *msg)
                 return -2; // can't correct errors
 
             mm->correctedbits = ei->errors;
+            mm->IID = 0;
             modesChecksumFix(msg, ei);
 
             // check whether the corrected message looks sensible