diff --git a/custom_components/versatile_thermostat/base_thermostat.py b/custom_components/versatile_thermostat/base_thermostat.py
index 4a8368f..0ce7f6a 100644
--- a/custom_components/versatile_thermostat/base_thermostat.py
+++ b/custom_components/versatile_thermostat/base_thermostat.py
@@ -1067,9 +1067,7 @@ class BaseThermostat(ClimateEntity, RestoreEntity, Generic[T]):
         save_state()
 
     @overrides
-    async def async_set_preset_mode(
-        self, preset_mode: str, overwrite_saved_preset=True
-    ):
+    async def async_set_preset_mode(self, preset_mode: str, overwrite_saved_preset=True):
         """Set new preset mode."""
 
         # We accept a new preset when:
@@ -1097,14 +1095,10 @@ class BaseThermostat(ClimateEntity, RestoreEntity, Generic[T]):
 
             return
 
-        await self.async_set_preset_mode_internal(
-            preset_mode, force=False, overwrite_saved_preset=overwrite_saved_preset
-        )
+        await self.async_set_preset_mode_internal(preset_mode, force=False, overwrite_saved_preset=overwrite_saved_preset)
         await self.async_control_heating(force=True)
 
-    async def async_set_preset_mode_internal(
-        self, preset_mode: str, force=False, overwrite_saved_preset=True
-    ):
+    async def async_set_preset_mode_internal(self, preset_mode: str, force=False, overwrite_saved_preset=True):
         """Set new preset mode."""
         _LOGGER.info("%s - Set preset_mode: %s force=%s", self, preset_mode, force)
         if (
@@ -1573,9 +1567,7 @@ class BaseThermostat(ClimateEntity, RestoreEntity, Generic[T]):
                 and HVACMode.HEAT in self.hvac_modes
             ):
                 await self.async_set_hvac_mode(HVACMode.HEAT)
-                await self.async_set_preset_mode(
-                    PRESET_FROST_PROTECTION, overwrite_saved_preset=False
-                )
+                await self.async_set_preset_mode(PRESET_FROST_PROTECTION, overwrite_saved_preset=False)
             else:
                 self.set_hvac_off_reason(HVAC_OFF_REASON_MANUAL)
                 await self.async_set_hvac_mode(HVACMode.OFF)
@@ -1800,9 +1792,7 @@ class BaseThermostat(ClimateEntity, RestoreEntity, Generic[T]):
         # If the changed preset is active, change the current temperature
         # Issue #119 - reload new preset temperature also in ac mode
         if preset.startswith(self._attr_preset_mode):
-            await self.async_set_preset_mode_internal(
-                preset.rstrip(PRESET_AC_SUFFIX), force=True
-            )
+            await self.async_set_preset_mode_internal(preset.rstrip(PRESET_AC_SUFFIX), force=True)
             await self.async_control_heating(force=True)
 
     async def SERVICE_SET_SAFETY(
diff --git a/custom_components/versatile_thermostat/thermostat_climate.py b/custom_components/versatile_thermostat/thermostat_climate.py
index f857a3a..276afae 100644
--- a/custom_components/versatile_thermostat/thermostat_climate.py
+++ b/custom_components/versatile_thermostat/thermostat_climate.py
@@ -3,7 +3,7 @@
 import logging
 from datetime import timedelta, datetime
 
-from homeassistant.const import STATE_ON
+from homeassistant.const import STATE_ON, STATE_UNAVAILABLE, STATE_UNKNOWN
 from homeassistant.core import Event, HomeAssistant, State, callback
 from homeassistant.helpers.event import (
     async_track_state_change_event,
@@ -614,7 +614,7 @@ class ThermostatOverClimate(BaseThermostat[UnderlyingClimate]):
             return
 
         # Find the underlying which have change
-        under = self.find_underlying_by_entity_id(new_state.entity_id)
+        under: UnderlyingClimate = self.find_underlying_by_entity_id(new_state.entity_id)
 
         if not under:
             _LOGGER.warning(
@@ -626,6 +626,16 @@ class ThermostatOverClimate(BaseThermostat[UnderlyingClimate]):
         new_hvac_mode = new_state.state
 
         old_state = event.data.get("old_state")
+
+        # Issue #829 - refresh underlying command if it comes back to life
+        if old_state is not None and new_state.state not in (STATE_UNAVAILABLE, STATE_UNKNOWN) and old_state.state in (STATE_UNAVAILABLE, STATE_UNKNOWN):
+            _LOGGER.warning("%s - underlying %s come back to life. New state=%s, old_state=%s. Will refresh its status", self, under.entity_id, new_state.state, old_state.state)
+            # Force hvac_mode and target temperature
+            await under.set_hvac_mode(self.hvac_mode)
+            await self._send_regulated_temperature(force=True)
+
+            return
+
         old_hvac_action = (
             old_state.attributes.get("hvac_action")
             if old_state and old_state.attributes
diff --git a/tests/commons.py b/tests/commons.py
index 3dde038..757080c 100644
--- a/tests/commons.py
+++ b/tests/commons.py
@@ -87,7 +87,6 @@ FULL_SWITCH_CONFIG = (
     | MOCK_TH_OVER_SWITCH_CENTRAL_MAIN_CONFIG
     | MOCK_TH_OVER_SWITCH_TYPE_CONFIG
     | MOCK_TH_OVER_SWITCH_TPI_CONFIG
-    #    | MOCK_PRESETS_CONFIG
     | MOCK_FULL_FEATURES
     | MOCK_WINDOW_CONFIG
     | MOCK_MOTION_CONFIG
@@ -111,12 +110,7 @@ FULL_SWITCH_AC_CONFIG = (
 )
 
 PARTIAL_CLIMATE_CONFIG = (
-    MOCK_TH_OVER_CLIMATE_USER_CONFIG
-    | MOCK_TH_OVER_CLIMATE_MAIN_CONFIG
-    | MOCK_TH_OVER_CLIMATE_CENTRAL_MAIN_CONFIG
-    | MOCK_TH_OVER_CLIMATE_TYPE_CONFIG
-    #    | MOCK_PRESETS_CONFIG
-    | MOCK_ADVANCED_CONFIG
+    MOCK_TH_OVER_CLIMATE_USER_CONFIG | MOCK_TH_OVER_CLIMATE_MAIN_CONFIG | MOCK_TH_OVER_CLIMATE_CENTRAL_MAIN_CONFIG | MOCK_TH_OVER_CLIMATE_TYPE_CONFIG | MOCK_ADVANCED_CONFIG
 )
 
 PARTIAL_CLIMATE_CONFIG_USE_DEVICE_TEMP = (
@@ -124,7 +118,6 @@ PARTIAL_CLIMATE_CONFIG_USE_DEVICE_TEMP = (
     | MOCK_TH_OVER_CLIMATE_MAIN_CONFIG
     | MOCK_TH_OVER_CLIMATE_CENTRAL_MAIN_CONFIG
     | MOCK_TH_OVER_CLIMATE_TYPE_USE_DEVICE_TEMP_CONFIG
-    #    | MOCK_PRESETS_CONFIG
     | MOCK_ADVANCED_CONFIG
 )
 
@@ -133,24 +126,17 @@ PARTIAL_CLIMATE_NOT_REGULATED_CONFIG = (
     | MOCK_TH_OVER_CLIMATE_MAIN_CONFIG
     | MOCK_TH_OVER_CLIMATE_CENTRAL_MAIN_CONFIG
     | MOCK_TH_OVER_CLIMATE_TYPE_NOT_REGULATED_CONFIG
-    #    | MOCK_PRESETS_CONFIG
     | MOCK_ADVANCED_CONFIG
 )
 
 PARTIAL_CLIMATE_AC_CONFIG = (
-    MOCK_TH_OVER_CLIMATE_USER_CONFIG
-    | MOCK_TH_OVER_CLIMATE_TYPE_AC_CONFIG
-    | MOCK_TH_OVER_CLIMATE_MAIN_CONFIG
-    | MOCK_TH_OVER_CLIMATE_CENTRAL_MAIN_CONFIG
-    #    | MOCK_PRESETS_CONFIG
-    | MOCK_ADVANCED_CONFIG
+    MOCK_TH_OVER_CLIMATE_USER_CONFIG | MOCK_TH_OVER_CLIMATE_TYPE_AC_CONFIG | MOCK_TH_OVER_CLIMATE_MAIN_CONFIG | MOCK_TH_OVER_CLIMATE_CENTRAL_MAIN_CONFIG | MOCK_ADVANCED_CONFIG
 )
 
 FULL_4SWITCH_CONFIG = (
     MOCK_TH_OVER_4SWITCH_USER_CONFIG
     | MOCK_TH_OVER_4SWITCH_TYPE_CONFIG
     | MOCK_TH_OVER_SWITCH_TPI_CONFIG
-    #    | MOCK_PRESETS_CONFIG
     | MOCK_WINDOW_CONFIG
     | MOCK_MOTION_CONFIG
     | MOCK_POWER_CONFIG
@@ -1037,6 +1023,16 @@ default_temperatures_ac_away = {
     "boost_ac_away": 23.1,
 }
 
+default_temperatures_ac = {
+    "frost": 7.0,
+    "eco": 17.0,
+    "comfort": 19.0,
+    "boost": 21.0,
+    "eco_ac": 27.0,
+    "comfort_ac": 25.0,
+    "boost_ac": 23.0,
+}
+
 default_temperatures_away = {
     "frost": 7.0,
     "eco": 17.0,
diff --git a/tests/test_overclimate.py b/tests/test_overclimate.py
index 30c27ce..ad317a5 100644
--- a/tests/test_overclimate.py
+++ b/tests/test_overclimate.py
@@ -1233,3 +1233,111 @@ async def test_manual_hvac_off_should_take_the_lead_over_auto_start_stop(
         assert vtherm.hvac_off_reason == HVAC_OFF_REASON_MANUAL
         assert vtherm._saved_hvac_mode == HVACMode.OFF
         assert mock_send_event.call_count == 0  # nothing have change
+
+
+@pytest.mark.parametrize("expected_lingering_tasks", [True])
+@pytest.mark.parametrize("expected_lingering_timers", [True])
+async def test_underlying_from_comes_back_to_life(
+    hass: HomeAssistant,
+    skip_hass_states_is_state,
+    skip_turn_on_off_heater,
+    skip_send_event,
+):
+    """Test that when a underlying climate comes back to life (from unkwown or unavailable) the last state is send"""
+
+    tz = get_tz(hass)  # pylint: disable=invalid-name
+    now: datetime = datetime.now(tz=tz)
+
+    entry = MockConfigEntry(
+        domain=DOMAIN,
+        title="TheOverClimateMockName",
+        unique_id="uniqueId",
+        data={
+            CONF_NAME: "TheOverClimateMockName",
+            CONF_TEMP_SENSOR: "sensor.mock_temp_sensor",
+            CONF_THERMOSTAT_TYPE: CONF_THERMOSTAT_CLIMATE,
+            CONF_EXTERNAL_TEMP_SENSOR: "sensor.mock_ext_temp_sensor",
+            CONF_CYCLE_MIN: 5,
+            CONF_TEMP_MIN: 15,
+            CONF_TEMP_MAX: 30,
+            CONF_USE_WINDOW_FEATURE: False,
+            CONF_USE_MOTION_FEATURE: False,
+            CONF_USE_POWER_FEATURE: False,
+            CONF_USE_AUTO_START_STOP_FEATURE: False,
+            CONF_USE_PRESENCE_FEATURE: False,
+            CONF_UNDERLYING_LIST: ["climate.mock_climate"],
+            CONF_MINIMAL_ACTIVATION_DELAY: 30,
+            CONF_SAFETY_DELAY_MIN: 5,
+            CONF_SAFETY_MIN_ON_PERCENT: 0.3,
+            CONF_AUTO_FAN_MODE: CONF_AUTO_FAN_NONE,
+            CONF_AC_MODE: True,
+        },  # 5 minutes security delay
+    )
+
+    # Underlying is in HEAT mode but should be shutdown at startup
+    fake_underlying_climate = MockClimate(hass, "mockUniqueId", "MockClimateName", {}, HVACMode.COOL, HVACAction.COOLING)
+
+    # 1. initialize the vtherm in COOL with Boost
+    # fmt: off
+    with patch("custom_components.versatile_thermostat.underlyings.UnderlyingClimate.find_underlying_climate",return_value=fake_underlying_climate) as mock_find_climate:
+    # fmt: on
+        entity = await create_thermostat(hass, entry, "climate.theoverclimatemockname", temps=default_temperatures_ac)
+
+        assert entity
+        assert entity.name == "TheOverClimateMockName"
+        assert entity.is_over_climate is True
+
+        # Set hvac_mode to COOL
+        await entity.async_set_hvac_mode(HVACMode.COOL)
+        await entity.async_set_preset_mode(PRESET_BOOST)
+
+        # it is very hot today
+        await send_temperature_change_event(entity, 27, now, False)
+        await send_ext_temperature_change_event(entity, 35, now, False)
+        await hass.async_block_till_done()
+
+        assert entity.hvac_mode is HVACMode.COOL
+        # because in MockClimate HVACAction is HEATING if hvac_mode is not set
+        assert entity.hvac_action is HVACAction.COOLING
+        assert entity.preset_mode is PRESET_BOOST
+        assert entity.target_temperature == 23
+
+
+    # 2. send under state event comes back from life
+    # fmt: off
+    with patch("custom_components.versatile_thermostat.underlyings.UnderlyingClimate.set_hvac_mode") as mock_underlying_set_hvac_mode, \
+        patch("custom_components.versatile_thermostat.underlyings.UnderlyingClimate.set_temperature") as mock_underlying_set_temperature:
+    # fmt: on
+        now  = now + timedelta(minutes=2)
+        # 2. Change the target temp of underlying thermostat at now -> the event will be disgarded because to fast (to avoid loop cf issue 121)
+        await send_climate_change_event_with_temperature(
+            entity,
+            HVACMode.HEAT,
+            STATE_UNKNOWN,
+            HVACAction.OFF,
+            STATE_UNKNOWN,
+            now,
+            entity.min_temp + 1,
+            True,
+            "climate.mock_climate",  # the underlying climate entity id
+        )
+
+        assert mock_underlying_set_hvac_mode.call_count == 1
+        mock_underlying_set_hvac_mode.assert_has_calls(
+            [
+                call.set_hvac_mode(HVACMode.COOL),
+            ]
+        )
+
+        assert mock_underlying_set_temperature.call_count == 1
+        mock_underlying_set_temperature.assert_has_calls(
+            [
+                call.set_temperature(23, 30, 15),
+            ]
+        )
+
+
+        # Nothing should have changed
+        assert entity.target_temperature == 23
+        assert entity.preset_mode is PRESET_BOOST
+        assert entity.hvac_mode is HVACMode.COOL