diff --git a/.github/workflows/CI-unittests.yml b/.github/workflows/CI-unittests.yml
index 9e01717163..b2c1499657 100644
--- a/.github/workflows/CI-unittests.yml
+++ b/.github/workflows/CI-unittests.yml
@@ -21,7 +21,44 @@ on:
     types: [published]
 
 jobs:
+  check_unit_test_files:
+    runs-on: ubuntu-latest
+
+    defaults:
+      run:
+        shell: bash -leo pipefail {0}
+
+    steps:
+      - name: checkout
+        uses: actions/checkout@v6
+        with:
+          fetch-depth: 0
+
+      - name: Python setup
+        uses: actions/setup-python@v6
+        with:
+          python-version: "3.12"
+          check-latest: true
+
+      - name: Check unit test files exist
+        run: |
+          python - <<'PY'
+          from pathlib import Path
+          src_root = Path("src/simtools")
+          test_root = Path("tests/unit_tests")
+          missing = []
+          for path in src_root.rglob("*.py"):
+              rel = path.relative_to(src_root)
+              if path.name in {"__init__.py", "_version.py"} or rel.parts[0] == "applications":
+                  continue
+              if not (test_root / rel.parent / f"test_{path.stem}.py").exists():
+                  missing.append(str(rel))
+          if missing:
+              raise SystemExit("Modules without unit tests:\n" + "\n".join(sorted(missing)))
+          PY
+
   unit_tests:
+    needs: check_unit_test_files
     runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
diff --git a/docs/changes/2193.maintenance.md b/docs/changes/2193.maintenance.md
new file mode 100644
index 0000000000..62bd813d0f
--- /dev/null
+++ b/docs/changes/2193.maintenance.md
@@ -0,0 +1 @@
+Add CI tests to ensure that every simtools Python module has a corresponding unit test module.
diff --git a/tests/unit_tests/configuration/test_defaults.py b/tests/unit_tests/configuration/test_defaults.py
new file mode 100644
index 0000000000..c105c4019d
--- /dev/null
+++ b/tests/unit_tests/configuration/test_defaults.py
@@ -0,0 +1,23 @@
+import simtools.configuration.defaults as defaults
+
+
+def test_corsika_defaults():
+    """Test default CORSIKA configuration constants."""
+    assert defaults.CORSIKA_PATH == "/workdir/simulation_software/corsika7"
+    assert (
+        defaults.CORSIKA_INTERACTION_TABLE_PATH
+        == "/workdir/external/simpipe/simulation_software/corsika7-interaction-tables/interaction-tables/"
+    )
+    assert defaults.CORSIKA_HE_INTERACTION == "epos"
+    assert defaults.CORSIKA_LE_INTERACTION == "urqmd"
+
+
+def test_simulation_software_defaults():
+    """Test simulation software default values."""
+    assert defaults.CURVED_ATMOSPHERE_MIN_ZENITH_ANGLE_DEG == 65
+    assert defaults.SIMULATION_SOFTWARE_CHOICES == (
+        "corsika",
+        "sim_telarray",
+        "corsika_sim_telarray",
+    )
+    assert defaults.SIMULATION_SOFTWARE_DEFAULT == "corsika_sim_telarray"
diff --git a/tests/unit_tests/production_configuration/test_calculate_statistical_uncertainties_grid.py b/tests/unit_tests/production_configuration/test_calculate_statistical_uncertainties_grid_point.py
similarity index 100%
rename from tests/unit_tests/production_configuration/test_calculate_statistical_uncertainties_grid.py
rename to tests/unit_tests/production_configuration/test_calculate_statistical_uncertainties_grid_point.py