diff --git a/automotive/vehicle/aidl/impl/fake_impl/hardware/include/FakeVehicleHardware.h b/automotive/vehicle/aidl/impl/fake_impl/hardware/include/FakeVehicleHardware.h
index 1636cb6..e515bad 100644
--- a/automotive/vehicle/aidl/impl/fake_impl/hardware/include/FakeVehicleHardware.h
+++ b/automotive/vehicle/aidl/impl/fake_impl/hardware/include/FakeVehicleHardware.h
@@ -138,9 +138,12 @@
     std::unique_ptr<RecurrentTimer> mRecurrentTimer;
     // GeneratorHub is thread-safe.
     std::unique_ptr<GeneratorHub> mGeneratorHub;
+
+    // Only allowed to set once.
+    std::unique_ptr<const PropertyChangeCallback> mOnPropertyChangeCallback;
+    std::unique_ptr<const PropertySetErrorCallback> mOnPropertySetErrorCallback;
+
     std::mutex mLock;
-    std::unique_ptr<const PropertyChangeCallback> mOnPropertyChangeCallback GUARDED_BY(mLock);
-    std::unique_ptr<const PropertySetErrorCallback> mOnPropertySetErrorCallback GUARDED_BY(mLock);
     std::unordered_map<PropIdAreaId, std::shared_ptr<RecurrentTimer::Callback>, PropIdAreaIdHash>
             mRecurrentActions GUARDED_BY(mLock);
     std::unordered_map<PropIdAreaId, VehiclePropValuePool::RecyclableType, PropIdAreaIdHash>
diff --git a/automotive/vehicle/aidl/impl/fake_impl/hardware/src/FakeVehicleHardware.cpp b/automotive/vehicle/aidl/impl/fake_impl/hardware/src/FakeVehicleHardware.cpp
index 736ecaa..dd76524 100644
--- a/automotive/vehicle/aidl/impl/fake_impl/hardware/src/FakeVehicleHardware.cpp
+++ b/automotive/vehicle/aidl/impl/fake_impl/hardware/src/FakeVehicleHardware.cpp
@@ -1228,13 +1228,19 @@
 
 void FakeVehicleHardware::registerOnPropertyChangeEvent(
         std::unique_ptr<const PropertyChangeCallback> callback) {
-    std::scoped_lock<std::mutex> lockGuard(mLock);
+    if (mOnPropertyChangeCallback != nullptr) {
+        ALOGE("registerOnPropertyChangeEvent must only be called once");
+        return;
+    }
     mOnPropertyChangeCallback = std::move(callback);
 }
 
 void FakeVehicleHardware::registerOnPropertySetErrorEvent(
         std::unique_ptr<const PropertySetErrorCallback> callback) {
-    std::scoped_lock<std::mutex> lockGuard(mLock);
+    if (mOnPropertySetErrorCallback != nullptr) {
+        ALOGE("registerOnPropertySetErrorEvent must only be called once");
+        return;
+    }
     mOnPropertySetErrorCallback = std::move(callback);
 }
 
@@ -1278,8 +1284,6 @@
 }
 
 void FakeVehicleHardware::onValueChangeCallback(const VehiclePropValue& value) {
-    std::scoped_lock<std::mutex> lockGuard(mLock);
-
     if (mOnPropertyChangeCallback == nullptr) {
         return;
     }
diff --git a/automotive/vehicle/aidl/impl/hardware/include/IVehicleHardware.h b/automotive/vehicle/aidl/impl/hardware/include/IVehicleHardware.h
index 759db41..d92ccfd 100644
--- a/automotive/vehicle/aidl/impl/hardware/include/IVehicleHardware.h
+++ b/automotive/vehicle/aidl/impl/hardware/include/IVehicleHardware.h
@@ -116,11 +116,12 @@
     virtual aidl::android::hardware::automotive::vehicle::StatusCode checkHealth() = 0;
 
     // Register a callback that would be called when there is a property change event from vehicle.
+    // Must only be called once during initialization.
     virtual void registerOnPropertyChangeEvent(
             std::unique_ptr<const PropertyChangeCallback> callback) = 0;
 
     // Register a callback that would be called when there is a property set error event from
-    // vehicle.
+    // vehicle. Must only be called once during initialization.
     virtual void registerOnPropertySetErrorEvent(
             std::unique_ptr<const PropertySetErrorCallback> callback) = 0;
 };