diff --git a/CHANGELOG.md b/CHANGELOG.md
index ef36f69b1..6c6cba7e2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,7 @@ All notable changes to this project will be documented in this file.
 This project uses the changelog in accordance with [keepchangelog](http://keepachangelog.com/). Please use this to write notable changes, which is not the same as git commit log...
 
 ## [unreleased][unreleased]
+ - Fix issue #844 - `lf t55xx config` => recompute block0 (@cyberpunk-re)
  - EM4x50: changed cli parameter from w (word) to d (data) (@tharexde)
  - EM4x50: new function 4x50 login: authenticate against tag (@tharexde)
  - EM4x50: new function 4x50 brute: guess password within a given password range (@tharexde)
@@ -21,7 +22,7 @@ This project uses the changelog in accordance with [keepchangelog](http://keepac
  - EM4x50: switched to cliparser for all functions (@tharexde)
  - EM4x50: stabilized and accelerated tag detection (@tharexde)
  - EM4x50: removed global tag structure on device side (@tharexde)
- - Fix `hf 15 sim` - Added basic response to GET_SYSTEM_INFO and READBLOCK requests in order to fix iso15693 tag sim
+ - Fix `hf 15 sim` - Added basic response to GET_SYSTEM_INFO and READBLOCK requests in order to fix iso15693 tag sim (@cyberpunk-re)
  - Added `mf mfu sim t 7 n <numreads>` - MFU emulation now supports automatic exit after <num> blocks read. (@cyberpunk-re)
  - Added T55xx Guide to assist in learning how to use the T55xx chip (@mwalker33)
  - Fix 'hf iclass wrbl' - dealing with tags in unsecured vs secured pagemode now is correct (@iceman1001)
diff --git a/client/src/cmdlft55xx.c b/client/src/cmdlft55xx.c
index 80c8eda4f..a3f5ca20b 100644
--- a/client/src/cmdlft55xx.c
+++ b/client/src/cmdlft55xx.c
@@ -51,6 +51,7 @@ t55xx_conf_block_t config = {
     .inverted = false,
     .offset = 0x00,
     .block0 = 0x00,
+    .block0Status = notSet,
     .Q5 = false,
     .usepwd = false,
     .downlink_mode = refFixedBit
@@ -739,6 +740,7 @@ static int CmdT55xxSetConfig(const char *Cmd) {
                     for (; i < 9; i++) {
                         if (rates[i] == bitRate) {
                             config.bitrate = i;
+                            config.block0 = ((config.block0 & ~(0x1c0000)) | (i << 18));
                             break;
                         }
                     }
@@ -789,6 +791,7 @@ static int CmdT55xxSetConfig(const char *Cmd) {
                     PrintAndLogEx(WARNING, "Unknown modulation '%s'", modulation);
                     errors = true;
                 }
+                config.block0 = ((config.block0 & ~(0x1f000)) | (config.modulation << 12));
                 break;
             case 'i':
                 if ((param_getchar(Cmd, cmdp + 1) == '0') || (param_getchar(Cmd, cmdp + 1) == '1')) {
@@ -822,6 +825,7 @@ static int CmdT55xxSetConfig(const char *Cmd) {
                     config.ST = true;
                     cmdp += 1;
                 }
+                config.block0 = ((config.block0 & ~(0x8)) | (config.ST << 3));
                 break;
             case 'r':
                 errors = param_getdec(Cmd, cmdp + 1, &downlink_mode);
@@ -841,10 +845,9 @@ static int CmdT55xxSetConfig(const char *Cmd) {
     //Validations
     if (errors) return usage_t55xx_config();
 
+    config.block0Status = userSet;
     if (gotconf) {
         SetConfigWithBlock0Ex(block0, config.offset, config.Q5);
-    } else {
-        config.block0 = 0;
     }
 
     return printConfiguration(config);
@@ -1335,6 +1338,7 @@ bool t55xxTryDetectModulationEx(uint8_t downlink_mode, bool print_config, uint32
             config.pwd = pwd & 0xffffffff;
         }
 
+        config.block0Status = autoDetect;
         if (print_config)
             printConfiguration(config);
 
@@ -1370,6 +1374,7 @@ bool t55xxTryDetectModulationEx(uint8_t downlink_mode, bool print_config, uint32
                 PrintAndLogEx(NORMAL, "--[%d]---------------", i + 1);
             }
 
+            config.block0Status = autoDetect;
             if (print_config)
                 printConfiguration(tests[i]);
         }
@@ -1640,7 +1645,7 @@ int printConfiguration(t55xx_conf_block_t b) {
     PrintAndLogEx(INFO, "     Inverted       : %s", (b.inverted) ? _GREEN_("Yes") : "No");
     PrintAndLogEx(INFO, "     Offset         : %d", b.offset);
     PrintAndLogEx(INFO, "     Seq. Term.     : %s", (b.ST) ? _GREEN_("Yes") : "No");
-    PrintAndLogEx(INFO, "     Block0         : 0x%08X", b.block0);
+    PrintAndLogEx(INFO, "     Block0         : 0x%08X %s", b.block0, GetConfigBlock0Source(b.block0Status));
     PrintAndLogEx(INFO, "     Downlink Mode  : %s", GetDownlinkModeStr(b.downlink_mode));
     PrintAndLogEx(INFO, "     Password Set   : %s", (b.usepwd) ? _RED_("Yes") : _GREEN_("No"));
     if (b.usepwd) {
@@ -2800,6 +2805,28 @@ char *GetModelStrFromCID(uint32_t cid) {
     return buf;
 }
 
+char *GetConfigBlock0Source(uint8_t id) {
+   
+    static char buf[40];
+    char *retStr = buf;
+
+    switch (id) {
+        case autoDetect:
+            snprintf(retStr, sizeof(buf), _YELLOW_("(Auto detect)"));
+            break;
+        case userSet:
+            snprintf(retStr, sizeof(buf), _YELLOW_("(User set)"));
+            break;
+        case tagRead:
+            snprintf(retStr, sizeof(buf), _GREEN_("(Tag read)"));
+            break;
+        default:
+            snprintf(retStr, sizeof(buf), _RED_("(Unknown)"));
+            break;
+    }
+    return buf;
+}
+
 char *GetSelectedModulationStr(uint8_t id) {
 
     static char buf[20];
diff --git a/client/src/cmdlft55xx.h b/client/src/cmdlft55xx.h
index 4352eed64..90fbaa85d 100644
--- a/client/src/cmdlft55xx.h
+++ b/client/src/cmdlft55xx.h
@@ -125,6 +125,12 @@ typedef struct {
     bool inverted;
     uint8_t offset;
     uint32_t block0;
+    enum {
+        notSet     = 0x00,
+        autoDetect = 0x01,
+        userSet    = 0x02,
+        tagRead    = 0x03, 
+    } block0Status;
     enum {
         RF_8 = 0x00,
         RF_16 = 0x01,
@@ -166,6 +172,7 @@ char *GetSaferStr(uint32_t id);
 char *GetQ5ModulationStr(uint32_t id);
 char *GetModulationStr(uint32_t id, bool xmode);
 char *GetModelStrFromCID(uint32_t cid);
+char *GetConfigBlock0Source(uint8_t id);
 char *GetSelectedModulationStr(uint8_t id);
 char *GetDownlinkModeStr(uint8_t downlink_mode);
 void printT5xxHeader(uint8_t page);