diff --git a/src/systems/abstractsystem.jl b/src/systems/abstractsystem.jl
index f114bc3fa7..216a83f85f 100644
--- a/src/systems/abstractsystem.jl
+++ b/src/systems/abstractsystem.jl
@@ -932,10 +932,11 @@ function complete(sys::AbstractSystem; split = true, flatten = true)
         @set! sys.ps = unique!(vcat(get_ps(sys), collect(newparams)))
     end
     if flatten
-        if sys isa Union{OptimizationSystem, ConstraintsSystem, JumpSystem}
-            newsys = sys
-        else
+        if (eqs = equations(sys)) isa Vector &&
+           any(eq -> eq isa Equation && isconnection(eq.lhs), eqs)
             newsys = expand_connections(sys)
+        else
+            newsys = sys
         end
         newsys = ModelingToolkit.flatten(newsys)
         if has_parent(newsys) && get_parent(sys) === nothing
diff --git a/test/runtests.jl b/test/runtests.jl
index 5db3308540..c7edbfaaf5 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -19,48 +19,48 @@ end
 @time begin
     if GROUP == "All" || GROUP == "InterfaceI"
         @testset "InterfaceI" begin
-            # @safetestset "Linear Algebra Test" include("linalg.jl")
-            # @safetestset "AbstractSystem Test" include("abstractsystem.jl")
-            # @safetestset "Variable Scope Tests" include("variable_scope.jl")
-            # @safetestset "Symbolic Parameters Test" include("symbolic_parameters.jl")
-            # @safetestset "Parsing Test" include("variable_parsing.jl")
-            # @safetestset "Simplify Test" include("simplify.jl")
-            # @safetestset "Direct Usage Test" include("direct.jl")
-            # @safetestset "System Linearity Test" include("linearity.jl")
-            # @safetestset "Input Output Test" include("input_output_handling.jl")
-            # @safetestset "Clock Test" include("clock.jl")
-            # @safetestset "ODESystem Test" include("odesystem.jl")
-            # @safetestset "Dynamic Quantities Test" include("dq_units.jl")
-            # @safetestset "Unitful Quantities Test" include("units.jl")
-            # @safetestset "Mass Matrix Test" include("mass_matrix.jl")
-            # @safetestset "InitializationSystem Test" include("initializationsystem.jl")
-            # @safetestset "Guess Propagation" include("guess_propagation.jl")
-            # @safetestset "Hierarchical Initialization Equations" include("hierarchical_initialization_eqs.jl")
-            # @safetestset "Reduction Test" include("reduction.jl")
-            # @safetestset "Split Parameters Test" include("split_parameters.jl")
-            # @safetestset "StaticArrays Test" include("static_arrays.jl")
-            # @safetestset "Components Test" include("components.jl")
-            # @safetestset "Model Parsing Test" include("model_parsing.jl")
-            # @safetestset "Error Handling" include("error_handling.jl")
-            # @safetestset "StructuralTransformations" include("structural_transformation/runtests.jl")
-            # @safetestset "State Selection Test" include("state_selection.jl")
-            # @safetestset "Symbolic Event Test" include("symbolic_events.jl")
-            # @safetestset "Stream Connect Test" include("stream_connectors.jl")
-            # @safetestset "Domain Connect Test" include("domain_connectors.jl")
-            # @safetestset "Lowering Integration Test" include("lowering_solving.jl")
-            # @safetestset "Dependency Graph Test" include("dep_graphs.jl")
-            # @safetestset "Function Registration Test" include("function_registration.jl")
-            # @safetestset "Precompiled Modules Test" include("precompile_test.jl")
-            # @safetestset "DAE Jacobians Test" include("dae_jacobian.jl")
-            # @safetestset "Jacobian Sparsity" include("jacobiansparsity.jl")
+            @safetestset "Linear Algebra Test" include("linalg.jl")
+            @safetestset "AbstractSystem Test" include("abstractsystem.jl")
+            @safetestset "Variable Scope Tests" include("variable_scope.jl")
+            @safetestset "Symbolic Parameters Test" include("symbolic_parameters.jl")
+            @safetestset "Parsing Test" include("variable_parsing.jl")
+            @safetestset "Simplify Test" include("simplify.jl")
+            @safetestset "Direct Usage Test" include("direct.jl")
+            @safetestset "System Linearity Test" include("linearity.jl")
+            @safetestset "Input Output Test" include("input_output_handling.jl")
+            @safetestset "Clock Test" include("clock.jl")
+            @safetestset "ODESystem Test" include("odesystem.jl")
+            @safetestset "Dynamic Quantities Test" include("dq_units.jl")
+            @safetestset "Unitful Quantities Test" include("units.jl")
+            @safetestset "Mass Matrix Test" include("mass_matrix.jl")
+            @safetestset "InitializationSystem Test" include("initializationsystem.jl")
+            @safetestset "Guess Propagation" include("guess_propagation.jl")
+            @safetestset "Hierarchical Initialization Equations" include("hierarchical_initialization_eqs.jl")
+            @safetestset "Reduction Test" include("reduction.jl")
+            @safetestset "Split Parameters Test" include("split_parameters.jl")
+            @safetestset "StaticArrays Test" include("static_arrays.jl")
+            @safetestset "Components Test" include("components.jl")
+            @safetestset "Model Parsing Test" include("model_parsing.jl")
+            @safetestset "Error Handling" include("error_handling.jl")
+            @safetestset "StructuralTransformations" include("structural_transformation/runtests.jl")
+            @safetestset "State Selection Test" include("state_selection.jl")
+            @safetestset "Symbolic Event Test" include("symbolic_events.jl")
+            @safetestset "Stream Connect Test" include("stream_connectors.jl")
+            @safetestset "Domain Connect Test" include("domain_connectors.jl")
+            @safetestset "Lowering Integration Test" include("lowering_solving.jl")
+            @safetestset "Dependency Graph Test" include("dep_graphs.jl")
+            @safetestset "Function Registration Test" include("function_registration.jl")
+            @safetestset "Precompiled Modules Test" include("precompile_test.jl")
+            @safetestset "DAE Jacobians Test" include("dae_jacobian.jl")
+            @safetestset "Jacobian Sparsity" include("jacobiansparsity.jl")
             @safetestset "Modelingtoolkitize Test" include("modelingtoolkitize.jl")
-            # @safetestset "FuncAffect Test" include("funcaffect.jl")
-            # @safetestset "Constants Test" include("constants.jl")
-            # @safetestset "Parameter Dependency Test" include("parameter_dependencies.jl")
-            # @safetestset "Generate Custom Function Test" include("generate_custom_function.jl")
-            # @safetestset "Initial Values Test" include("initial_values.jl")
-            # @safetestset "Equation Type Accessors Test" include("equation_type_accessors.jl")
-            # @safetestset "Equations with complex values" include("complex.jl")
+            @safetestset "FuncAffect Test" include("funcaffect.jl")
+            @safetestset "Constants Test" include("constants.jl")
+            @safetestset "Parameter Dependency Test" include("parameter_dependencies.jl")
+            @safetestset "Generate Custom Function Test" include("generate_custom_function.jl")
+            @safetestset "Initial Values Test" include("initial_values.jl")
+            @safetestset "Equation Type Accessors Test" include("equation_type_accessors.jl")
+            @safetestset "Equations with complex values" include("complex.jl")
         end
     end