diff --git a/phosphor-regulators/src/meson.build b/phosphor-regulators/src/meson.build
index d9ba765..581343f 100644
--- a/phosphor-regulators/src/meson.build
+++ b/phosphor-regulators/src/meson.build
@@ -17,6 +17,7 @@
     'ffdc_file.cpp',
     'id_map.cpp',
     'journal.cpp',
+    'phase_fault_detection.cpp',
     'pmbus_utils.cpp',
     'presence_detection.cpp',
     'presence_service.cpp',
diff --git a/phosphor-regulators/src/phase_fault_detection.cpp b/phosphor-regulators/src/phase_fault_detection.cpp
new file mode 100644
index 0000000..4ca276b
--- /dev/null
+++ b/phosphor-regulators/src/phase_fault_detection.cpp
@@ -0,0 +1,144 @@
+/**
+ * Copyright © 2021 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 "phase_fault_detection.hpp"
+
+#include "action_utils.hpp"
+#include "chassis.hpp"
+#include "device.hpp"
+#include "error_logging.hpp"
+#include "error_logging_utils.hpp"
+#include "exception_utils.hpp"
+#include "journal.hpp"
+#include "system.hpp"
+
+#include <exception>
+#include <map>
+
+namespace phosphor::power::regulators
+{
+
+/**
+ * Maximum number of action errors to write to the journal.
+ */
+constexpr unsigned short maxActionErrorCount{3};
+
+/**
+ * Number of consecutive phase faults required to log an error.  This provides
+ * "de-glitching" to ignore transient hardware problems.
+ */
+constexpr unsigned short requiredConsecutiveFaults{2};
+
+void PhaseFaultDetection::execute(Services& services, System& system,
+                                  Chassis& /*chassis*/, Device& regulator)
+{
+    try
+    {
+        // Find the device ID to use.  If the deviceID data member is empty, use
+        // the ID of the specified regulator.
+        const std::string& effectiveDeviceID =
+            deviceID.empty() ? regulator.getID() : deviceID;
+
+        // Create ActionEnvironment
+        ActionEnvironment environment{system.getIDMap(), effectiveDeviceID,
+                                      services};
+
+        // Execute the actions to detect phase faults
+        action_utils::execute(actions, environment);
+
+        // Check for any N or N+1 phase faults that were detected
+        checkForPhaseFault(PhaseFaultType::n, services, regulator, environment);
+        checkForPhaseFault(PhaseFaultType::n_plus_1, services, regulator,
+                           environment);
+    }
+    catch (const std::exception& e)
+    {
+        // Log error messages in journal if we haven't hit the max
+        if (actionErrorCount < maxActionErrorCount)
+        {
+            ++actionErrorCount;
+            services.getJournal().logError(exception_utils::getMessages(e));
+            services.getJournal().logError(
+                "Unable to detect phase faults in regulator " +
+                regulator.getID());
+        }
+
+        // Create error log entry if this type hasn't already been logged
+        error_logging_utils::logError(std::current_exception(),
+                                      Entry::Level::Warning, services,
+                                      errorHistory);
+    }
+}
+
+void PhaseFaultDetection::checkForPhaseFault(PhaseFaultType faultType,
+                                             Services& services,
+                                             Device& regulator,
+                                             ActionEnvironment& environment)
+{
+    // Find ErrorType that corresponds to PhaseFaultType; used by ErrorHistory
+    ErrorType errorType = toErrorType(faultType);
+
+    // If this error has not been logged yet
+    if (!errorHistory.wasLogged(errorType))
+    {
+        // Create reference to consecutive fault count data member
+        unsigned short& faultCount =
+            (faultType == PhaseFaultType::n) ? nFaultCount : nPlus1FaultCount;
+
+        // Check if the phase fault was detected
+        if (environment.getPhaseFaults().count(faultType) == 0)
+        {
+            // Phase fault not detected; reset consecutive fault count
+            faultCount = 0;
+        }
+        else
+        {
+            // Phase fault detected; increment consecutive fault count
+            ++faultCount;
+
+            // Log error message in journal
+            services.getJournal().logError(
+                toString(faultType) + " phase fault detected in regulator " +
+                regulator.getID() + ": count=" + std::to_string(faultCount));
+
+            // If the required number of consecutive faults have been detected
+            if (faultCount >= requiredConsecutiveFaults)
+            {
+                // Log phase fault error and update ErrorHistory
+                logPhaseFault(faultType, services, regulator, environment);
+                errorHistory.setWasLogged(errorType, true);
+            }
+        }
+    }
+}
+
+void PhaseFaultDetection::logPhaseFault(PhaseFaultType faultType,
+                                        Services& services, Device& regulator,
+                                        ActionEnvironment& environment)
+{
+    ErrorLogging& errorLogging = services.getErrorLogging();
+    Entry::Level severity = (faultType == PhaseFaultType::n)
+                                ? Entry::Level::Warning
+                                : Entry::Level::Informational;
+    Journal& journal = services.getJournal();
+    const std::string& inventoryPath = regulator.getFRU();
+    const std::map<std::string, std::string>& additionalData =
+        environment.getAdditionalErrorData();
+    errorLogging.logPhaseFault(severity, journal, faultType, inventoryPath,
+                               additionalData);
+}
+
+} // namespace phosphor::power::regulators
diff --git a/phosphor-regulators/src/phase_fault_detection.hpp b/phosphor-regulators/src/phase_fault_detection.hpp
new file mode 100644
index 0000000..467d8c7
--- /dev/null
+++ b/phosphor-regulators/src/phase_fault_detection.hpp
@@ -0,0 +1,208 @@
+/**
+ * Copyright © 2021 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.
+ */
+#pragma once
+
+#include "action.hpp"
+#include "action_environment.hpp"
+#include "error_history.hpp"
+#include "phase_fault.hpp"
+#include "services.hpp"
+
+#include <memory>
+#include <string>
+#include <utility>
+#include <vector>
+
+namespace phosphor::power::regulators
+{
+
+// Forward declarations to avoid circular dependencies
+class Chassis;
+class Device;
+class System;
+
+/**
+ * @class PhaseFaultDetection
+ *
+ * Detects and logs redundant phase faults in a voltage regulator.
+ *
+ * A voltage regulator is sometimes called a "phase controller" because it
+ * controls one or more phases that perform the actual voltage regulation.
+ *
+ * A regulator may have redundant phases.  If a redundant phase fails, the
+ * regulator will continue to provide the desired output voltage.  However, a
+ * phase fault error should be logged warning the user that the regulator has
+ * lost redundancy.
+ *
+ * The technique used to detect a phase fault varies depending on the regulator
+ * hardware.  Often a bit is checked in a status register.  The status register
+ * could exist in the regulator or in a related I/O expander.
+ *
+ * Phase fault detection is performed every 15 seconds.  A phase fault must be
+ * detected two consecutive times (15 seconds apart) before an error is logged.
+ * This provides "de-glitching" to ignore transient hardware problems.
+ *
+ * Phase faults are detected by executing actions.
+ */
+class PhaseFaultDetection
+{
+  public:
+    // Specify which compiler-generated methods we want
+    PhaseFaultDetection() = delete;
+    PhaseFaultDetection(const PhaseFaultDetection&) = delete;
+    PhaseFaultDetection(PhaseFaultDetection&&) = delete;
+    PhaseFaultDetection& operator=(const PhaseFaultDetection&) = delete;
+    PhaseFaultDetection& operator=(PhaseFaultDetection&&) = delete;
+    ~PhaseFaultDetection() = default;
+
+    /**
+     * Constructor.
+     *
+     * @param actions Actions that detect phase faults in the regulator.
+     * @param deviceID Unique ID of the device to use when detecting phase
+     *                 faults.  If not specified, the regulator will be used.
+     */
+    explicit PhaseFaultDetection(std::vector<std::unique_ptr<Action>> actions,
+                                 const std::string& deviceID = "") :
+        actions{std::move(actions)},
+        deviceID{deviceID}
+    {
+    }
+
+    /**
+     * Clears all error history.
+     *
+     * All data on previously logged errors will be deleted.  If errors occur
+     * again in the future they will be logged again.
+     *
+     * This method is normally called when the system is being powered on.
+     */
+    void clearErrorHistory()
+    {
+        errorHistory.clear();
+        actionErrorCount = 0;
+        nFaultCount = 0;
+        nPlus1FaultCount = 0;
+    }
+
+    /**
+     * Executes the actions that detect phase faults in the regulator.
+     *
+     * If the required number of consecutive phase faults are detected, an error
+     * is logged.
+     *
+     * @param services system services like error logging and the journal
+     * @param system system that contains the chassis
+     * @param chassis chassis that contains the regulator device
+     * @param regulator voltage regulator device
+     */
+    void execute(Services& services, System& system, Chassis& chassis,
+                 Device& regulator);
+
+    /**
+     * Returns the actions that detect phase faults in the regulator.
+     *
+     * @return actions
+     */
+    const std::vector<std::unique_ptr<Action>>& getActions() const
+    {
+        return actions;
+    }
+
+    /**
+     * Returns the unique ID of the device to use when detecting phase
+     * faults.
+     *
+     * If the value is "", the regulator will be used.
+     *
+     * @return device ID
+     */
+    const std::string& getDeviceID() const
+    {
+        return deviceID;
+    }
+
+  private:
+    /**
+     * Checks if the specified phase fault type was detected.
+     *
+     * If the fault type was detected, increments the counter tracking
+     * consecutive faults.  If the required number of consecutive faults have
+     * been detected, logs a phase fault error.
+     *
+     * The ActionEnvironment contains the set of phase fault types that were
+     * detected (if any).
+     *
+     * @param faultType phase fault type to check
+     * @param services system services like error logging and the journal
+     * @param regulator voltage regulator device
+     * @param environment action execution environment
+     */
+    void checkForPhaseFault(PhaseFaultType faultType, Services& services,
+                            Device& regulator, ActionEnvironment& environment);
+
+    /**
+     * Logs an error for the specified phase fault type.
+     *
+     * @param faultType phase fault type that occurred
+     * @param services system services like error logging and the journal
+     * @param regulator voltage regulator device
+     * @param environment action execution environment
+     */
+    void logPhaseFault(PhaseFaultType faultType, Services& services,
+                       Device& regulator, ActionEnvironment& environment);
+
+    /**
+     * Actions that detect phase faults in the regulator.
+     */
+    std::vector<std::unique_ptr<Action>> actions{};
+
+    /**
+     * Unique ID of the device to use when detecting phase faults.
+     *
+     * Sometimes a separate device, such as an I/O expander, is accessed to
+     * obtain the phase fault status for a regulator.
+     *
+     * If the value is "", the regulator will be used.
+     */
+    const std::string deviceID{};
+
+    /**
+     * History of which error types have been logged.
+     *
+     * Since phase fault detection runs repeatedly based on a timer, each error
+     * type is only logged once.
+     */
+    ErrorHistory errorHistory{};
+
+    /**
+     * Number of errors that have occurred while executing actions, resulting in
+     * an exception.
+     */
+    unsigned short actionErrorCount{0};
+
+    /**
+     * Number of consecutive N phase faults that have been detected.
+     */
+    unsigned short nFaultCount{0};
+
+    /**
+     * Number of consecutive N+1 phase faults that have been detected.
+     */
+    unsigned short nPlus1FaultCount{0};
+};
+
+} // namespace phosphor::power::regulators