Index: firmware/App/Modes/FPModes/ModePreGenPermeate.c
===================================================================
diff -u -rdc883aab56e769b2dec294e084f1378a178ba089 -rf0aec0f46d5e85250430ecc95e4654200cf2ed83
--- firmware/App/Modes/FPModes/ModePreGenPermeate.c	(.../ModePreGenPermeate.c)	(revision dc883aab56e769b2dec294e084f1378a178ba089)
+++ firmware/App/Modes/FPModes/ModePreGenPermeate.c	(.../ModePreGenPermeate.c)	(revision f0aec0f46d5e85250430ecc95e4654200cf2ed83)
@@ -18,6 +18,7 @@
 #include "BoostPump.h"
 #include "FPModeStandby.h"
 #include "FPOperationModes.h"
+#include "Level.h"
 #include "MessageSupport.h"
 #include "Messaging.h"
 #include "ModeGenPermeate.h"
@@ -549,4 +550,89 @@
     return result;
 }
 
+/*********************************************************************//**
+ * @brief
+ * The testValidatePreGenPChangeReq function validate whether requested
+ * sub mode change is legal or not.
+ * @details \b Inputs: none
+ * @details \b Outputs: none
+ * @param newSubMode the enumerated sub state of the mode.
+ * @return TRUE if request is legal, FALSE if not.
+ *************************************************************************/
+BOOL testValidatePreGenPChangeReq( U32 newSubMode )
+{
+    BOOL result = FALSE;
+    LEVEL_STATE_T level = getLevelStatus( P25_LEVL );
+
+    if ( newSubMode < NUM_OF_FP_PRE_GENW_MODE_STATES )
+    {
+        switch ( newSubMode )
+        {
+            case FP_PRE_GENP_CONCENTRATE_FLUSH:
+                result = (( LEVEL_STATE_HIGH == level ) ? TRUE : FALSE);
+                break;
+
+            case FP_PRE_GENP_INLET_PRESSURE_CHECK:
+            case FP_PRE_GENP_FILTER_FLUSH:
+            case FP_PRE_GENP_PERMEATE_FLUSH:
+            case FP_PRE_GENP_VERIFY_WATER:
+                result = TRUE;
+                break;
+
+            // TODO change once pause state is implemented
+            case FP_PRE_GENP_PAUSED:
+            default:
+                break;
+        }
+    }
+
+    return result;
+}
+
+/*********************************************************************//**
+ * @brief
+ * The testSignalNewPreGenPSubMode function requests transition to a new
+ * PreGen Permeate sub mode.
+ * @details \b Inputs: none
+ * @details \b Outputs: preGenPState
+ * @param newSubMode the enumerated sub state of the mode.
+ * @return none
+ *************************************************************************/
+void testSignalNewPreGenPSubMode( U32 newSubMode )
+{
+    if ( newSubMode < NUM_OF_FP_PRE_GENW_MODE_STATES )
+    {
+        preGenPState = (FP_PRE_GENP_MODE_STATE_T)newSubMode;
+        // handle the prerequisite if any
+        switch ( newSubMode )
+        {
+            case FP_PRE_GENP_INLET_PRESSURE_CHECK:
+                resetInletPressureCheckSignals();
+                break;
+
+            case FP_PRE_GENP_FILTER_FLUSH:
+                resetFilterFlushSignals();
+                signalStartFilterFlush();
+                break;
+
+            case FP_PRE_GENP_PERMEATE_FLUSH:
+                resetPermeateFlushSignals();
+                signalStartPermeateFlush();
+                break;
+
+            case FP_PRE_GENP_CONCENTRATE_FLUSH:
+                resetConcentrateFlushSignals();
+                signalStartConcentrateFlush();
+                break;
+
+            // Require no special handling for now (update if required in future)
+            case FP_PRE_GENP_VERIFY_WATER:
+            case FP_PRE_GENP_PAUSED:
+            default:
+                break;
+        }
+        setModePreGenPTransition( preGenPState );
+    }
+}
+
 /**@}*/
Index: firmware/App/Services/Messaging.c
===================================================================
diff -u -r214bc36051314945ae212a1869aa87014eb1dbf8 -rf0aec0f46d5e85250430ecc95e4654200cf2ed83
--- firmware/App/Services/Messaging.c	(.../Messaging.c)	(revision 214bc36051314945ae212a1869aa87014eb1dbf8)
+++ firmware/App/Services/Messaging.c	(.../Messaging.c)	(revision f0aec0f46d5e85250430ecc95e4654200cf2ed83)
@@ -279,11 +279,11 @@
     { MSG_ID_FP_DEF_STATUS_REQUEST,                                                 &testGetFPDefeaturedStatus },
     { MSG_ID_FP_RO_FILTERED_REJECTION_RATIO_OVERRIDE_REQUEST,                       &testRORejectionRatioFilteredOverride },
     { MSG_ID_FP_SET_RECOVERY_VALVES_REQUEST,                                        &testIOFPSetValveRecoveryConfig },
-
     { MSG_ID_DD_BICARB_DOSE_VOL_CONTROL_KP_GAIN_COEFF_OVERRIDE_REQUEST,             &testDryBiCartBicarbDoseVolControlKpGainOverride },
     { MSG_ID_DD_BICARB_DOSE_VOL_CONTROL_KI_GAIN_COEFF_OVERRIDE_REQUEST,             &testDryBiCartBicarbDoseVolControlKiGainOverride },
     { MSG_ID_DD_ACID_DOSE_VOL_CONTROL_KP_GAIN_COEFF_OVERRIDE_REQUEST,               &testDryBiCartAcidDoseVolControlKpGainOverride },
     { MSG_ID_DD_ACID_DOSE_VOL_CONTROL_KI_GAIN_COEFF_OVERRIDE_REQUEST,               &testDryBiCartAcidDoseVolControlKiGainOverride },
+    { MSG_ID_FP_SET_OPERATION_SUB_MODE_REQUEST,                                     &testSetFPOperationSubMode },
 };
 
 /// Calculation for number of entries in the incoming message function handler look-up table.