diff --git a/presence/fallback.cpp b/presence/fallback.cpp
new file mode 100644
index 0000000..625d74a
--- /dev/null
+++ b/presence/fallback.cpp
@@ -0,0 +1,101 @@
+/**
+ * Copyright © 2017 IBM Corporation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#include <algorithm>
+#include <phosphor-logging/log.hpp>
+#include "fan.hpp"
+#include "fallback.hpp"
+#include "psensor.hpp"
+
+namespace phosphor
+{
+namespace fan
+{
+namespace presence
+{
+
+void Fallback::stateChanged(bool present)
+{
+    if (!present)
+    {
+        // Starting with the first backup, find the first
+        // sensor that reports the fan as present, if any.
+        auto it = std::find_if(
+                std::next(activeSensor),
+                sensors.end(),
+                [](auto& s){return s.get().present();});
+
+        if (it != sensors.end())
+        {
+            // A backup sensor disagrees with the active sensor.
+            // Switch to the backup.
+            activeSensor->get().stop();
+            present = it->get().start();
+
+            while (activeSensor != it)
+            {
+                // Callout the broken sensors.
+                activeSensor->get().fail();
+                ++activeSensor;
+            }
+            phosphor::logging::log<phosphor::logging::level::INFO>(
+                    "Using backup presence sensor.",
+                    phosphor::logging::entry(
+                        "FAN=%s", std::get<1>(fan)));
+            activeSensor = it;
+        }
+    }
+
+    setPresence(fan, present);
+}
+
+void Fallback::monitor()
+{
+    // Find the first sensor that says the fan is present
+    // and set it as the active sensor.
+    activeSensor = std::find_if(
+            sensors.begin(),
+            sensors.end(),
+            [](auto& s){return s.get().present();});
+    if (activeSensor == sensors.end())
+    {
+        // The first sensor is working or all sensors
+        // agree the fan isn't present.  Use the first sensor.
+        activeSensor = sensors.begin();
+    }
+
+    if (activeSensor != sensors.begin())
+    {
+        phosphor::logging::log<phosphor::logging::level::INFO>(
+                "Using backup presence sensor.",
+                phosphor::logging::entry(
+                    "FAN=%s", std::get<1>(fan)));
+    }
+
+    // Callout the broken sensors.
+    auto it = sensors.begin();
+    while (it != activeSensor)
+    {
+        it->get().fail();
+        ++it;
+    }
+
+    // Start the active sensor and set the initial state.
+    setPresence(fan, activeSensor->get().start());
+}
+
+} // namespace presence
+} // namespace fan
+} // namespace phosphor