diff --git a/src/services/ui.lua b/src/services/ui.lua
index 1d60cdf0..f91975aa 100644
--- a/src/services/ui.lua
+++ b/src/services/ui.lua
@@ -10,7 +10,7 @@ local websocket = require "http.websocket"
 local cjson = require "cjson.safe"
 local op = require "fibers.op"
 local sleep = require "fibers.sleep"
--- -@type { get_box_reports: fun(): table, get_box_logs: fun(): table }
+---@type { get_box_reports: fun(config: table, gsm_stats: table|nil): table, get_box_logs: fun(): table }
 local diagnostics = require "services.ui.diagnostics"
 require "services.ui.fibers_cqueues"
 
@@ -207,7 +207,7 @@ local function onstream(self, stream)
 
             if req_method == "GET" then
                 ---@diagnostic disable-next-line: need-check-nil
-                local diagnostics_reports = diagnostics.get_box_reports(config)
+                local diagnostics_reports = diagnostics.get_box_reports(config, stats_messages_cache)
                 ---@diagnostic disable-next-line: need-check-nil
                 local diagnostics_logs = diagnostics.get_box_logs()
                 local resp = { diagnostics_logs = diagnostics_logs, diagnostics = diagnostics_reports }
diff --git a/src/services/ui/diagnostics.lua b/src/services/ui/diagnostics.lua
index 24d19c8f..ede90226 100644
--- a/src/services/ui/diagnostics.lua
+++ b/src/services/ui/diagnostics.lua
@@ -351,20 +351,60 @@ local function check_expected_cloud_services_reachable(box_type, config)
         end
     end
 
-    return {
-        expected  = expected,
-        installed = installed,
-        missing   = missing
-    }
+    return new_report(expected, installed, missing)
+end
+
+---Check which modems are active (SIM present and state machine running)
+---@param box_type string Box model type
+---@param stats_cache table|nil Flat stats messages cache
+---@return table<string, number|string[]>|nil report
+---@return string|nil error
+local function get_modems_sim_active(box_type, stats_cache)
+    local expected_modems, err = expected.get_expected_modem_names(box_type)
+    if err ~= nil then
+        return nil, err
+    end
+
+    local installed = 0
+    local missing = {}
+
+    if not stats_cache then
+        return new_report(#expected_modems, 0, expected_modems), nil
+    end
+
+    for _, modem_name in ipairs(expected_modems) do
+        local sim_key = "gsm.modem." .. modem_name .. ".sim"
+        local sim_entry = stats_cache[sim_key]
+        local sim_present = sim_entry and sim_entry.payload == "present"
+
+        local state_key = "gsm.modem." .. modem_name .. ".state"
+        local state_entry = stats_cache[state_key]
+        local has_active_state = state_entry
+            and state_entry.payload
+            and state_entry.payload.curr_state
+            and state_entry.payload.curr_state ~= ""
+
+        local is_active = sim_present and has_active_state
+
+        if is_active then
+            installed = installed + 1
+        else
+            table.insert(missing, modem_name)
+        end
+    end
+
+    return new_report(#expected_modems, installed, missing), nil
 end
 
 ---Fetches diagnostics stats from the box
----@return table<string, table> diagnostics
 ---@param config table<string, string> The configuration
-local function get_box_reports(config)
+---@param stats_cache table|nil Flat stats messages cache
+---@return table<string, table> diagnostics
+local function get_box_reports(config, stats_cache)
     local diagnostics = {
         packages_installed = {},
         modems_installed = {},
+        modems_sim_active = {},
         services_running = {},
         packages_running = {},
         bootstrap_installed = {},
@@ -404,6 +444,14 @@ local function get_box_reports(config)
         else
             diagnostics.cloud_services_reachable = cloud_services
         end
+
+        -- Check modem SIM active (present + state machine running)
+        local modems_sim_active, sim_check_err = get_modems_sim_active(hardware_info.model, stats_cache)
+        if sim_check_err ~= nil then
+            log.error("UI - error checking modem SIM active", sim_check_err)
+        else
+            diagnostics.modems_sim_active = modems_sim_active
+        end
     else
         log.error("UI - error getting hardware info", hardware_info_err)
     end
diff --git a/src/services/ui/diagnostics_expected.lua b/src/services/ui/diagnostics_expected.lua
index 5d84d69e..bc0e7a57 100644
--- a/src/services/ui/diagnostics_expected.lua
+++ b/src/services/ui/diagnostics_expected.lua
@@ -166,6 +166,10 @@ local GETBOX_MODEMS = 1
 local BIGBOX_SS_MODEMS = 2
 local BIGBOX_V1_CM_MODEMS = 2
 
+local GETBOX_MODEM_NAMES = {"primary"}
+local BIGBOX_SS_MODEM_NAMES = {"primary", "secondary"}
+local BIGBOX_V1_CM_MODEM_NAMES = {"primary", "secondary"}
+
 local BOOTSTRAP_INSTALLED = {
     "/data/configs/hawkbit.cfg",
     "/data/configs/mainflux.cfg",
@@ -301,11 +305,28 @@ local function get_expected_modem_count(box_type)
     end
 end
 
+---Get the expected modem names for the box model
+---@param box_type string getbox|bigbox-ss|bigbox-v1-cm
+---@return string[] expected_modem_names
+---@return string|nil error
+local function get_expected_modem_names(box_type)
+    if box_type == GETBOX then
+        return GETBOX_MODEM_NAMES, nil
+    elseif box_type == BIGBOX_SS then
+        return BIGBOX_SS_MODEM_NAMES, nil
+    elseif box_type == BIGBOX_V1_CM then
+        return BIGBOX_V1_CM_MODEM_NAMES, nil
+    else
+        return {}, "Unknown box_type: " .. box_type
+    end
+end
+
 return {
     packages_running = PACKAGES_RUNNING,
     bootstrap_installed = BOOTSTRAP_INSTALLED,
     get_expected_packages_installed = get_expected_packages_installed,
     get_expected_services_running = get_expected_services_running,
     get_expected_modem_count = get_expected_modem_count,
+    get_expected_modem_names = get_expected_modem_names,
     get_expected_connectivity_tests = get_expected_connectivity_tests,
 }