diff --git a/src/HaHelper.h b/src/HaHelper.h
index 3258f0e..8583d40 100644
--- a/src/HaHelper.h
+++ b/src/HaHelper.h
@@ -885,13 +885,33 @@ public:
     doc[FPSTR(HA_NAME)] = F("Fault code");
     doc[FPSTR(HA_ICON)] = F("mdi:chat-alert-outline");
     doc[FPSTR(HA_STATE_TOPIC)] = this->getDeviceTopic(F("state"));
-    doc[FPSTR(HA_VALUE_TEMPLATE)] = F("{{ \"E%02d\"|format(value_json.sensors.faultCode) }}");
+    doc[FPSTR(HA_VALUE_TEMPLATE)] = F("{{ \"%02d (0x%02X)\"|format(value_json.sensors.faultCode, value_json.sensors.faultCode) }}");
     doc[FPSTR(HA_EXPIRE_AFTER)] = 120;
     doc.shrinkToFit();
 
     return this->publish(this->getTopic(FPSTR(HA_ENTITY_SENSOR), F("fault_code")).c_str(), doc);
   }
 
+  bool publishSensorDiagnosticCode(bool enabledByDefault = true) {
+    JsonDocument doc;
+    doc[FPSTR(HA_AVAILABILITY)][0][FPSTR(HA_TOPIC)] = this->getDeviceTopic(F("status"));
+    doc[FPSTR(HA_AVAILABILITY)][1][FPSTR(HA_TOPIC)] = this->getDeviceTopic(F("state"));
+    doc[FPSTR(HA_AVAILABILITY)][1][FPSTR(HA_VALUE_TEMPLATE)] = F("{{ iif(value_json.states.otStatus and value_json.states.fault or value_json.states.diagnostic, 'online', 'offline') }}");
+    doc[FPSTR(HA_AVAILABILITY_MODE)] = F("all");
+    doc[FPSTR(HA_ENABLED_BY_DEFAULT)] = enabledByDefault;
+    doc[FPSTR(HA_UNIQUE_ID)] = this->getObjectId(F("diagnostic_code"));
+    doc[FPSTR(HA_OBJECT_ID)] = this->getObjectId(F("diagnostic_code"));
+    doc[FPSTR(HA_ENTITY_CATEGORY)] = F("diagnostic");
+    doc[FPSTR(HA_NAME)] = F("Diagnostic code");
+    doc[FPSTR(HA_ICON)] = F("mdi:chat-alert-outline");
+    doc[FPSTR(HA_STATE_TOPIC)] = this->getDeviceTopic(F("state"));
+    doc[FPSTR(HA_VALUE_TEMPLATE)] = F("{{ \"%02d (0x%02X)\"|format(value_json.sensors.diagnosticCode, value_json.sensors.diagnosticCode) }}");
+    doc[FPSTR(HA_EXPIRE_AFTER)] = 120;
+    doc.shrinkToFit();
+
+    return this->publish(this->getTopic(FPSTR(HA_ENTITY_SENSOR), F("diagnostic_code")).c_str(), doc);
+  }
+
   bool publishSensorRssi(bool enabledByDefault = true) {
     JsonDocument doc;
     doc[FPSTR(HA_AVAILABILITY)][FPSTR(HA_TOPIC)] = this->getDeviceTopic(F("status"));
diff --git a/src/MqttTask.h b/src/MqttTask.h
index 479d800..49e3761 100644
--- a/src/MqttTask.h
+++ b/src/MqttTask.h
@@ -358,6 +358,7 @@ protected:
     this->haHelper->publishSensorModulation(false);
     this->haHelper->publishSensorPressure(settings.system.unitSystem, false);
     this->haHelper->publishSensorFaultCode();
+    this->haHelper->publishSensorDiagnosticCode();
     this->haHelper->publishSensorRssi(false);
     this->haHelper->publishSensorUptime(false);
 
diff --git a/src/OpenThermTask.h b/src/OpenThermTask.h
index 3c58da9..3e77713 100644
--- a/src/OpenThermTask.h
+++ b/src/OpenThermTask.h
@@ -347,6 +347,14 @@ protected:
         vars.sensors.faultCode = 0;
       }
 
+      // Get diagnostic code (if necessary)
+      if (vars.states.fault || vars.states.diagnostic) {
+        updateDiagCode();
+        
+      } else if (vars.sensors.diagnosticCode != 0) {
+        vars.sensors.diagnosticCode = 0;
+      }
+
       updatePressure();
 
       this->prevUpdateNonEssentialVars = millis();
@@ -933,6 +941,7 @@ protected:
     ));
 
     if (!CustomOpenTherm::isValidResponse(response)) {
+      vars.sensors.faultCode = 0;
       return false;
     }
 
@@ -940,6 +949,22 @@ protected:
     return true;
   }
 
+  bool updateDiagCode() {
+    unsigned long response = this->instance->sendRequest(CustomOpenTherm::buildRequest(
+      OpenThermRequestType::READ_DATA,
+      OpenThermMessageID::OEMDiagnosticCode,
+      0
+    ));
+
+    if (!CustomOpenTherm::isValidResponse(response)) {
+      vars.sensors.diagnosticCode = 0;
+      return false;
+    }
+
+    vars.sensors.diagnosticCode = CustomOpenTherm::getUInt(response);
+    return true;
+  }
+
   bool updateModulationLevel() {
     unsigned long response = this->instance->sendRequest(CustomOpenTherm::buildRequest(
       OpenThermRequestType::READ_DATA,
diff --git a/src/Settings.h b/src/Settings.h
index dba3e78..333ff02 100644
--- a/src/Settings.h
+++ b/src/Settings.h
@@ -168,6 +168,7 @@ struct Variables {
     float pressure = 0.0f;
     float dhwFlowRate = 0.0f;
     byte faultCode = 0;
+    unsigned short diagnosticCode = 0;
     int8_t rssi = 0;
 
     struct {
diff --git a/src/utils.h b/src/utils.h
index b966f94..4a77629 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -1507,6 +1507,7 @@ void varsToJson(const Variables& src, JsonVariant dst) {
   dst["sensors"]["pressure"] = roundd(src.sensors.pressure, 2);
   dst["sensors"]["dhwFlowRate"] = roundd(src.sensors.dhwFlowRate, 2);
   dst["sensors"]["faultCode"] = src.sensors.faultCode;
+  dst["sensors"]["diagnosticCode"] = src.sensors.diagnosticCode;
   dst["sensors"]["rssi"] = src.sensors.rssi;
   dst["sensors"]["uptime"] = millis() / 1000ul;
   dst["sensors"]["outdoor"]["connected"] = src.sensors.outdoor.connected;
diff --git a/src_data/locales/en.json b/src_data/locales/en.json
index d4595fc..d3df819 100644
--- a/src_data/locales/en.json
+++ b/src_data/locales/en.json
@@ -100,6 +100,7 @@
         "pressure": "Pressure",
         "dhwFlowRate": "DHW flow rate",
         "faultCode": "Fault code",
+        "diagCode": "Diagnostic code",
         "indoorTemp": "Indoor temp",
         "outdoorTemp": "Outdoor temp",
         "heatingTemp": "Heating temp",
diff --git a/src_data/locales/ru.json b/src_data/locales/ru.json
index 0637077..2e22763 100644
--- a/src_data/locales/ru.json
+++ b/src_data/locales/ru.json
@@ -100,6 +100,7 @@
         "pressure": "Давление",
         "dhwFlowRate": "Расход ГВС",
         "faultCode": "Код ошибки",
+        "diagCode": "Диагностический код",
         "indoorTemp": "Внутренняя темп.",
         "outdoorTemp": "Наружная темп.",
         "heatingTemp": "Темп. отопления",
diff --git a/src_data/pages/dashboard.html b/src_data/pages/dashboard.html
index 4f05b15..d2f48c8 100644
--- a/src_data/pages/dashboard.html
+++ b/src_data/pages/dashboard.html
@@ -162,6 +162,10 @@
                   <th scope="row" data-i18n>dashboard.state.faultCode</th>
                   <td><b id="ot-fault-code"></b></td>
                 </tr>
+                <tr></tr>
+                  <th scope="row" data-i18n>dashboard.state.diagCode</th>
+                  <td><b id="ot-diag-code"></b></td>
+                </tr>
                 <tr>
                   <th scope="row" data-i18n>dashboard.state.indoorTemp</th>
                   <td><b id="indoor-temp"></b> <span class="temp-unit"></span></td>
@@ -430,7 +434,18 @@
             setValue('#ot-modulation', result.sensors.modulation);
             setValue('#ot-pressure', result.sensors.pressure);
             setValue('#ot-dhw-flow-rate', result.sensors.dhwFlowRate);
-            setValue('#ot-fault-code', result.sensors.faultCode ? ("E" + result.sensors.faultCode) : "-");
+            setValue(
+              '#ot-fault-code',
+              result.sensors.faultCode
+                ? (result.sensors.faultCode + " (0x" + dec2hex(result.sensors.faultCode) + ")")
+                : "-"
+            );
+            setValue(
+              '#ot-diag-code',
+              result.sensors.diagnosticCode
+                ? (result.sensors.diagnosticCode + " (0x" + dec2hex(result.sensors.diagnosticCode) + ")")
+                : "-"
+            );
 
             setValue('#indoor-temp', result.temperatures.indoor);
             setValue('#outdoor-temp', result.temperatures.outdoor);
diff --git a/src_data/scripts/utils.js b/src_data/scripts/utils.js
index c7944ec..d89358a 100644
--- a/src_data/scripts/utils.js
+++ b/src_data/scripts/utils.js
@@ -665,4 +665,13 @@ function form2json(data, noCastItems = []) {
 
   let object = Array.from(data).reduce(method, {});
   return JSON.stringify(object);
+}
+
+function dec2hex(i) {
+  let hex = parseInt(i).toString(16);
+  if (hex.length % 2 != 0) {
+    hex = "0" + hex;
+  }
+  
+  return hex.toUpperCase();
 }
\ No newline at end of file