diff --git a/lightsim2grid/tests/test_solver_control.py b/lightsim2grid/tests/test_solver_control.py
index eb5eca5..aa1a441 100644
--- a/lightsim2grid/tests/test_solver_control.py
+++ b/lightsim2grid/tests/test_solver_control.py
@@ -14,9 +14,10 @@
 # - change slack weight X
 # - turnoff_gen_pv X
 # - when it diverges (and then I can make it un converge normally) X
-# - test change bus to -1 and deactivate element has the same impact
+# - test change bus to -1 and deactivate element has the same impact (irrelevant !)
+# - test when set_bus to 2
 
-# TODO and do that for all solver type: NR, NRSingleSlack, GaussSeidel, GaussSeidelSynch, FDPF_XB and FDPFBX
+# TODO and do that for all solver type: NR X, NRSingleSlack, GaussSeidel, GaussSeidelSynch, FDPF_XB and FDPFBX
 # and for all solver check everything that can be check: final tolerance, number of iteration, etc. etc.
 
 import unittest
@@ -43,9 +44,9 @@ def setUp(self) -> None:
                                     action_class=CompleteAction,
                                     backend=LightSimBackend())
         self.gridmodel = self.env.backend._grid
-        self._aux_setup_grid()
-        self.v_init = 1.0 * self.env.backend.V
         self.iter = 10
+        self._aux_setup_grid()
+        self.v_init = 0.0 * self.env.backend.V + 1.04  # just to have a vector with the right dimension
         self.tol_solver = 1e-8  # solver
         self.tol_equal = 1e-10  #  for comparing with and without the "smarter solver" things, and make sure everything is really equal!
     
@@ -586,6 +587,166 @@ def test_divergence_ybus_dc(self):
         """test I can make the grid diverge and converge again using ybus (islanding) in DC"""  
         self.test_divergence_ybus_ac("_run_dc_pf")   
         
+    def _aux_disco_load(self, gridmodel, el_id, el_val):
+        gridmodel.deactivate_load(el_id)
+        
+    def _aux_reco_load(self, gridmodel, el_id, el_val):
+        gridmodel.reactivate_load(el_id)
+        
+    def test_disco_reco_load_ac(self, runpf_fun="_run_ac_pf"):
+        """test I can disconnect a load (AC)"""
+        for load in self.gridmodel.get_loads():
+            self.gridmodel.tell_solver_need_reset()
+            expected_diff = 1e-2
+            if load.id == 3:
+                expected_diff = 3e-3
+            self.aux_do_undo_ac(funname_do="_aux_disco_load",
+                                funname_undo="_aux_reco_load",
+                                runpf_fun=runpf_fun,
+                                el_id=load.id,
+                                expected_diff=expected_diff,
+                                el_val=0,
+                                to_add_remove=0,
+                                )
+
+    def test_disco_reco_load_dc(self):   
+        """test I can disconnect a load (DC)"""
+        if not self.need_dc:
+            self.skipTest("Useless to run DC")
+        self.test_disco_reco_load_ac(runpf_fun="_run_dc_pf")
+        
+    def _aux_disco_gen(self, gridmodel, el_id, el_val):
+        gridmodel.deactivate_gen(el_id)
+        
+    def _aux_reco_gen(self, gridmodel, el_id, el_val):
+        gridmodel.reactivate_gen(el_id)
+        
+    def test_disco_reco_gen_ac(self, runpf_fun="_run_ac_pf"):
+        """test I can disconnect a gen (AC)"""
+        for gen in self.gridmodel.get_generators():
+            if gen.is_slack:
+                # by default single slack, so I don't disconnect it
+                continue
+            if gen.target_p_mw == 0.:
+                # will have not impact as by default gen with p==0. are still pv
+                continue
+            self.gridmodel.tell_solver_need_reset()
+            expected_diff = 1e-2
+            if gen.id == 3:
+                expected_diff = 3e-3
+            self.aux_do_undo_ac(funname_do="_aux_disco_gen",
+                                funname_undo="_aux_reco_gen",
+                                runpf_fun=runpf_fun,
+                                el_id=gen.id,
+                                expected_diff=expected_diff,
+                                el_val=0,
+                                to_add_remove=0,
+                                )
+
+    def test_disco_reco_gen_dc(self):   
+        """test I can disconnect a shunt (DC)"""
+        if not self.need_dc:
+            self.skipTest("Useless to run DC")
+        self.test_disco_reco_gen_ac(runpf_fun="_run_dc_pf")
+        
+    def _aux_disco_shunt(self, gridmodel, el_id, el_val):
+        gridmodel.deactivate_shunt(el_id)
+        
+    def _aux_reco_shunt(self, gridmodel, el_id, el_val):
+        gridmodel.reactivate_shunt(el_id)
+        
+    def test_disco_reco_shunt_ac(self, runpf_fun="_run_ac_pf"):
+        """test I can disconnect a shunt (AC)"""
+        for shunt in self.gridmodel.get_shunts():
+            self.gridmodel.tell_solver_need_reset()
+            expected_diff = 1e-2
+            if runpf_fun == "_run_dc_pf":
+                # in dc q is not used, so i skipped if no target_p_mw
+                if shunt.target_p_mw == 0:
+                    continue
+            self.aux_do_undo_ac(funname_do="_aux_disco_shunt",
+                                funname_undo="_aux_reco_shunt",
+                                runpf_fun=runpf_fun,
+                                el_id=shunt.id,
+                                expected_diff=expected_diff,
+                                el_val=0,
+                                to_add_remove=0,
+                                )
+
+    def test_disco_reco_shunt_dc(self):   
+        """test I can disconnect a shunt (DC)"""
+        if not self.need_dc:
+            self.skipTest("Useless to run DC")
+        self.test_disco_reco_shunt_ac(runpf_fun="_run_dc_pf")
+    
+    def _aux_disco_shunt(self, gridmodel, el_id, el_val):
+        gridmodel.deactivate_bus(0)
+        gridmodel.reactivate_bus(15)
+        gridmodel.change_bus_powerline_or(0, 15)
+        gridmodel.change_bus_powerline_or(1, 15)
+        gridmodel.change_bus_gen(5, 15)
+        
+    def _aux_reco_shunt(self, gridmodel, el_id, el_val):
+        gridmodel.reactivate_bus(0)
+        gridmodel.deactivate_bus(15)
+        gridmodel.change_bus_powerline_or(0, 0)
+        gridmodel.change_bus_powerline_or(1, 0)
+        gridmodel.change_bus_gen(5, 0)
         
+    def test_change_bus2_ac(self, runpf_fun="_run_ac_pf"):
+        """test for bus 2, basic test I don't do it for all kind of objects (AC pf)"""
+        expected_diff = 1e-2
+        self.aux_do_undo_ac(funname_do="_aux_disco_shunt",
+                            funname_undo="_aux_reco_shunt",
+                            runpf_fun=runpf_fun,
+                            el_id=0,
+                            expected_diff=expected_diff,
+                            el_val=0,
+                            to_add_remove=0,
+                            )
+        
+ 
+class TestSolverControlNRSing(TestSolverControl):
+    def _aux_setup_grid(self):
+        self.need_dc = False  # is it worth it to run DC powerflow ?
+        self.can_dist_slack = False
+        self.gridmodel.change_solver(SolverType.SparseLUSingleSlack)
+        self.gridmodel.change_solver(SolverType.DC)
+ 
+ 
+class TestSolverControlFDPF_XB(TestSolverControl):
+    def _aux_setup_grid(self):
+        self.need_dc = False  # is it worth it to run DC powerflow ?
+        self.can_dist_slack = False
+        self.gridmodel.change_solver(SolverType.FDPF_XB_SparseLU)
+        self.gridmodel.change_solver(SolverType.DC)
+        self.iter = 30
+ 
+ 
+class TestSolverControlFDPF_BX(TestSolverControl):
+    def _aux_setup_grid(self):
+        self.need_dc = False  # is it worth it to run DC powerflow ?
+        self.can_dist_slack = False
+        self.gridmodel.change_solver(SolverType.FDPF_BX_SparseLU)
+        self.gridmodel.change_solver(SolverType.DC)
+        self.iter = 30
+               
+ 
+class TestSolverControlGaussSeidel(TestSolverControl):
+    def _aux_setup_grid(self):
+        self.need_dc = False  # is it worth it to run DC powerflow ?
+        self.can_dist_slack = False
+        self.gridmodel.change_solver(SolverType.GaussSeidel)
+        self.iter = 350
+ 
+ 
+class TestSolverControlGaussSeidelSynch(TestSolverControl):
+    def _aux_setup_grid(self):
+        self.need_dc = False  # is it worth it to run DC powerflow ?
+        self.can_dist_slack = False
+        self.gridmodel.change_solver(SolverType.GaussSeidelSynch)
+        self.iter = 1000
+               
+               
 if __name__ == "__main__":
     unittest.main()
diff --git a/src/GridModel.h b/src/GridModel.h
index b972d73..123a711 100644
--- a/src/GridModel.h
+++ b/src/GridModel.h
@@ -717,8 +717,6 @@ class GridModel : public GenericContainer
                     {
                         (this->*fun_react)(el_id); // eg reactivate_load(load_id);
                         (this->*fun_change)(el_id, new_bus_backend); // eg change_bus_load(load_id, new_bus_backend);
-                        // topo_changed_ = true;
-                        solver_control_.tell_dimension_changed();
                     }
                 } else{
                     if(has_changed(el_pos))
@@ -728,8 +726,6 @@ class GridModel : public GenericContainer
                         // bus_status_ is set to "false" in GridModel.update_topo
                         // and a bus is activated if (and only if) one element is connected to it.
                         // I must not set `bus_status_[new_bus_backend] = false;` in this case !
-                        // topo_changed_ = true;
-                        solver_control_.tell_dimension_changed();
                     }
                 }
             }
diff --git a/src/element_container/ShuntContainer.h b/src/element_container/ShuntContainer.h
index 2cbed98..4fe808c 100644
--- a/src/element_container/ShuntContainer.h
+++ b/src/element_container/ShuntContainer.h
@@ -131,13 +131,15 @@ class ShuntContainer : public GenericContainer
 
     void deactivate(int shunt_id, SolverControl & solver_control) {
         if(status_[shunt_id]){
-            solver_control.tell_recompute_sbus();
+            solver_control.tell_recompute_sbus();  // DC
+            solver_control.tell_recompute_ybus();  // AC
         }
         _deactivate(shunt_id, status_);
     }
     void reactivate(int shunt_id, SolverControl & solver_control) {
-        if(status_[shunt_id]){
-            solver_control.tell_recompute_sbus();
+        if(!status_[shunt_id]){
+            solver_control.tell_recompute_sbus();  // DC
+            solver_control.tell_recompute_ybus();  // AC
         }
         _reactivate(shunt_id, status_);
     }
diff --git a/src/powerflow_algorithm/BaseFDPFAlgo.tpp b/src/powerflow_algorithm/BaseFDPFAlgo.tpp
index fe13caf..28a7c1b 100644
--- a/src/powerflow_algorithm/BaseFDPFAlgo.tpp
+++ b/src/powerflow_algorithm/BaseFDPFAlgo.tpp
@@ -33,20 +33,30 @@ bool BaseFDPFAlgo<LinearSolver, XB_BX>::compute_pf(const Eigen::SparseMatrix<cpl
         // TODO DEBUG MODE
         std::ostringstream exc_;
         exc_ << "BaseFDPFAlgo::compute_pf: Size of the Sbus should be the same as the size of Ybus. Currently: ";
-        exc_ << "Sbus  (" << Sbus.size() << ") and Ybus (" << Ybus.rows() << ", " << Ybus.rows() << ").";
+        exc_ << "Sbus  (" << Sbus.size() << ") and Ybus (" << Ybus.rows() << ", " << Ybus.cols() << ").";
         throw std::runtime_error(exc_.str());
     }
     if(V.size() != Ybus.rows() || V.size() != Ybus.cols() ){
         // TODO DEBUG MODE
         std::ostringstream exc_;
         exc_ << "BaseFDPFAlgo::compute_pf: Size of V (init voltages) should be the same as the size of Ybus. Currently: ";
-        exc_ << "V  (" << V.size() << ") and Ybus (" << Ybus.rows()<< ", " << Ybus.rows() << ").";
+        exc_ << "V  (" << V.size() << ") and Ybus (" << Ybus.rows()<< ", " << Ybus.cols() << ").";
         throw std::runtime_error(exc_.str());
     }
     reset_timer();
-    auto timer = CustTimer();
     if(!is_linear_solver_valid()) return false;
+    if(_solver_control.need_reset_solver() || 
+       _solver_control.has_dimension_changed() ||
+       _solver_control.ybus_change_sparsity_pattern() ||
+       _solver_control.has_ybus_some_coeffs_zero() ||
+       _solver_control.has_slack_participate_changed() ||
+       _solver_control.has_pv_changed() ||
+       _solver_control.has_pq_changed()){
+       reset();
+    }
+
     err_ = ErrorType::NoError;  // reset the error if previous error happened
+    auto timer = CustTimer();
 
     Eigen::VectorXi my_pv = retrieve_pv_with_slack(slack_ids, pv);  // retrieve_pv_with_slack (not all), add_slack_to_pv (all)
     real_type slack_absorbed = std::real(Sbus.sum());  // initial guess for slack_absorbed