diff --git a/homeassistant/components/sensor/min_max.py b/homeassistant/components/sensor/min_max.py
index d612ca5cf26..506163d1c70 100644
--- a/homeassistant/components/sensor/min_max.py
+++ b/homeassistant/components/sensor/min_max.py
@@ -98,7 +98,7 @@ class MinMaxSensor(Entity):
             if self._unit_of_measurement != new_state.attributes.get(
                     ATTR_UNIT_OF_MEASUREMENT):
                 _LOGGER.warning("Units of measurement do not match")
-                return
+
             try:
                 self.states[entity] = float(new_state.state)
             except ValueError:
@@ -150,10 +150,36 @@ class MinMaxSensor(Entity):
         """Get the latest data and updates the states."""
         sensor_values = [self.states[k] for k in self._entity_ids
                          if k in self.states]
-        if len(sensor_values) == self.count_sensors:
-            self.min_value = min(sensor_values)
-            self.max_value = max(sensor_values)
-            self.mean = round(sum(sensor_values) / self.count_sensors,
-                              self._round_digits)
-        else:
-            self.min_value = self.max_value = self.mean = STATE_UNKNOWN
+        self.min_value = self.calc_min(sensor_values)
+        self.max_value = self.calc_max(sensor_values)
+        self.mean = self.calc_mean(sensor_values)
+
+    def calc_min(self, sensor_values):
+        """Calculate min value, honoring unkown states."""
+        val = STATE_UNKNOWN
+        for v in sensor_values:
+            if v != STATE_UNKNOWN:
+                if val == STATE_UNKNOWN or val > v:
+                    val = v
+        return val
+
+    def calc_max(self, sensor_values):
+        """Calculate max value, honoring unkown states."""
+        val = STATE_UNKNOWN
+        for v in sensor_values:
+            if v != STATE_UNKNOWN:
+                if val == STATE_UNKNOWN or val < v:
+                    val = v
+        return val
+
+    def calc_mean(self, sensor_values):
+        """Calculate mean value, honoring unkown states."""
+        val = 0
+        count = 0
+        for v in sensor_values:
+            if v != STATE_UNKNOWN:
+                val += v
+                count += 1
+        if count == 0:
+            return STATE_UNKNOWN
+        return round(val/count, self._round_digits)
diff --git a/tests/components/sensor/test_min_max.py b/tests/components/sensor/test_min_max.py
index b610775b39b..be140127b45 100644
--- a/tests/components/sensor/test_min_max.py
+++ b/tests/components/sensor/test_min_max.py
@@ -2,8 +2,7 @@
 import unittest
 
 from homeassistant.setup import setup_component
-from homeassistant.const import (
-    STATE_UNKNOWN, ATTR_UNIT_OF_MEASUREMENT, TEMP_CELSIUS, TEMP_FAHRENHEIT)
+from homeassistant.const import STATE_UNKNOWN
 from tests.common import get_test_home_assistant
 
 
@@ -191,62 +190,20 @@ class TestMinMaxSensor(unittest.TestCase):
 
         entity_ids = config['sensor']['entity_ids']
 
-        self.hass.states.set(entity_ids[0], self.values[0])
+        self.hass.states.set(entity_ids[0], STATE_UNKNOWN)
         self.hass.block_till_done()
 
         state = self.hass.states.get('sensor.test_max')
         self.assertEqual(STATE_UNKNOWN, state.state)
 
-        self.hass.states.set(entity_ids[1], self.values[1])
-        self.hass.block_till_done()
-
-        state = self.hass.states.get('sensor.test_max')
-        self.assertEqual(STATE_UNKNOWN, state.state)
-
-        self.hass.states.set(entity_ids[2], self.values[2])
+        self.hass.states.set(entity_ids[2], self.values[1])
         self.hass.block_till_done()
 
         state = self.hass.states.get('sensor.test_max')
         self.assertNotEqual(STATE_UNKNOWN, state.state)
 
-    def test_different_unit_of_measurement(self):
-        """Test for different unit of measurement."""
-        config = {
-            'sensor': {
-                'platform': 'min_max',
-                'name': 'test',
-                'type': 'mean',
-                'entity_ids': [
-                    'sensor.test_1',
-                    'sensor.test_2',
-                    'sensor.test_3',
-                ]
-            }
-        }
-
-        assert setup_component(self.hass, 'sensor', config)
-
-        entity_ids = config['sensor']['entity_ids']
-
-        self.hass.states.set(entity_ids[0], self.values[0],
-                             {ATTR_UNIT_OF_MEASUREMENT: TEMP_CELSIUS})
+        self.hass.states.set(entity_ids[2], STATE_UNKNOWN)
         self.hass.block_till_done()
 
-        state = self.hass.states.get('sensor.test_mean')
-
-        self.assertEqual(STATE_UNKNOWN, state.state)
-        self.assertEqual('°C', state.attributes.get('unit_of_measurement'))
-
-        self.hass.states.set(entity_ids[1], self.values[1],
-                             {ATTR_UNIT_OF_MEASUREMENT: TEMP_FAHRENHEIT})
-        self.hass.block_till_done()
-
-        self.assertEqual(STATE_UNKNOWN, state.state)
-        self.assertEqual('°C', state.attributes.get('unit_of_measurement'))
-
-        self.hass.states.set(entity_ids[2], self.values[2],
-                             {ATTR_UNIT_OF_MEASUREMENT: '%'})
-        self.hass.block_till_done()
-
-        self.assertEqual(STATE_UNKNOWN, state.state)
-        self.assertEqual('°C', state.attributes.get('unit_of_measurement'))
+        state = self.hass.states.get('sensor.test_max')
+        self.assertNotEqual(STATE_UNKNOWN, state.state)