diff --git a/lib/ModelingToolkitBase/src/systems/abstractsystem.jl b/lib/ModelingToolkitBase/src/systems/abstractsystem.jl
index 31d5deec18..00d57653f9 100644
--- a/lib/ModelingToolkitBase/src/systems/abstractsystem.jl
+++ b/lib/ModelingToolkitBase/src/systems/abstractsystem.jl
@@ -1998,6 +1998,20 @@ equations during `mtkcompile`. These equations matches generated numerical code.
 See also [`equations`](@ref) and [`ModelingToolkitBase.get_eqs`](@ref).
 """
 function full_equations(sys::AbstractSystem; simplify = false)
+    subsys = get_systems(sys)
+    # Fast path using `IRInfo`
+    if isempty(subsys)
+        new_eqs = Equation[]
+        eqs = equations(sys)
+        sizehint!(new_eqs, length(eqs))
+        info = get_ir_info(sys)
+        ir = get_irstructure(sys)
+        @assert length(info.eqs_idxs) == length(eqs)
+        for (eq, rhs_idx) in zip(eqs, info.eqs_idxs)
+            push!(new_eqs, eq.lhs ~ ir[rhs_idx])
+        end
+        return new_eqs
+    end
     empty_substitutions(sys) && return equations(sys)
     subs = get_substitutions(sys)
     neweqs = map(equations(sys)) do eq
diff --git a/lib/ModelingToolkitBase/test/serialization.jl b/lib/ModelingToolkitBase/test/serialization.jl
index 9bffd12f96..16814a6830 100644
--- a/lib/ModelingToolkitBase/test/serialization.jl
+++ b/lib/ModelingToolkitBase/test/serialization.jl
@@ -29,8 +29,7 @@ str = String(take!(io))
 
 sys = include_string(@__MODULE__, str)
 rc2 = expand_connections(rc_model)
-@test isapprox(sys, rc2)
-@test issetequal(equations(sys), equations(rc2))
+@test issetequal(full_equations(sys), full_equations(rc2))
 @test issetequal(unknowns(sys), unknowns(rc2))
 @test issetequal(parameters(sys), parameters(rc2))
 
@@ -54,5 +53,4 @@ sol_ = solve(prob_, ImplicitEuler())
 probexpr = ODEProblem{true}(ss, [capacitor.v => 0.0], (0, 0.1); expr = Val{true}, missing_guess_value);
 prob_obs = eval(probexpr)
 sol_obs = solve(prob_obs, ImplicitEuler())
-@show all_obs
 @test sol_obs[all_obs] == sol[all_obs]
diff --git a/src/problems/sccnonlinearproblem.jl b/src/problems/sccnonlinearproblem.jl
index 4c16426d72..15aebcdcb1 100644
--- a/src/problems/sccnonlinearproblem.jl
+++ b/src/problems/sccnonlinearproblem.jl
@@ -296,7 +296,10 @@ function build_caches!(sys::System, decomposition::SCCDecomposition)
             push!(banned_vars, split_indexed_var(u)[1])
         end
 
-        _eqs = get_eqs(subsys)
+        # While we own the system and so mutation _should_ be safe, `IRInfo` exists.
+        # It stores the indices corresponding to equations in the `IRStructure`, which
+        # would be incorrect if we mutate.
+        _eqs = copy(get_eqs(subsys))
         exprs_to_search = SymbolicT[]
         for i in eachindex(_eqs)
             push!(exprs_to_search, _eqs[i].rhs)
@@ -306,6 +309,7 @@ function build_caches!(sys::System, decomposition::SCCDecomposition)
         for i in eachindex(_eqs)
             _eqs[i] = _eqs[i].lhs ~ subber(_eqs[i].rhs)
         end
+        subsys = decomposition.subsystems[i] = ConstructionBase.setproperties(subsys; eqs = _eqs)
 
         if decomposition.islinear[i]
             store_to_mutable_cache!(subsys, CachedLinearAb, nothing)