diff --git a/tests/utils/fixtures/legacy_unknown_keys.json b/tests/utils/fixtures/legacy_unknown_keys.json
new file mode 100644
index 0000000..86f32c5
--- /dev/null
+++ b/tests/utils/fixtures/legacy_unknown_keys.json
@@ -0,0 +1,78 @@
+{
+	"ambient_temperature": 20,
+	"log_interval": 300,
+	"TempCtrl": {
+		"pid_type": "Smith",
+		"beta": 0.05,
+		"Outer": {
+			"kp": 0.4,
+			"ki": 0.0,
+			"kd": 0.0,
+			"kt": 0.0,
+			"y_hold_min": -0.1
+		},
+		"Inner": {
+			"Heat": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5
+			},
+			"Hold": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5,
+				"yi_max": 0.3
+			},
+			"Cool": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5
+			}
+		},
+		"Thresholds": {
+			"HoldHeat": 1.0,
+			"HoldCool": 1.0,
+			"HeatHold": 1.0,
+			"HeatCool": 1.0,
+			"CoolHold": 1.0,
+			"CoolHeat": 1.0
+		},
+		"Kalman": {
+			"var_P": 1.0,
+			"var_Q": 0.01,
+			"var_R": 0.1
+		},
+		"Model": {
+			"kn": 0.0,
+			"gain": 0.8
+		}
+	},
+	"Heater": {
+		"type": "sim"
+	},
+	"Stirrer": {
+		"type": "sim"
+	},
+	"TempSensor": {
+		"type": "sim",
+		"temp_offset": 0.0,
+		"sigma": 0.05,
+		"stirrer_sigma": 0.0,
+		"stirrer_tau": 20.0
+	},
+	"Pot": {
+		"mass": 5.96,
+		"material": "Edelstahl 18/10",
+		"L": 0.2,
+		"Td": 17,
+		"water_mass": 22,
+		"volumen": 30
+	},
+	"Plant": {
+		"kn": 0.0,
+		"gain": 0.8
+	}
+}
diff --git a/tests/utils/fixtures/missing_inner_hold_branch.json b/tests/utils/fixtures/missing_inner_hold_branch.json
new file mode 100644
index 0000000..ffed149
--- /dev/null
+++ b/tests/utils/fixtures/missing_inner_hold_branch.json
@@ -0,0 +1,58 @@
+{
+	"ambient_temperature": 20,
+	"log_interval": 300,
+	"TempCtrl": {
+		"pid_type": "Smith",
+		"beta": 0.05,
+		"Outer": {
+			"kp": 0.4,
+			"ki": 0.0,
+			"kd": 0.0,
+			"kt": 0.0,
+			"y_hold_min": -0.1
+		},
+		"Inner": {
+			"Heat": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5
+			},
+			"Cool": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5
+			}
+		},
+		"Thresholds": {
+			"HoldHeat": 1.0,
+			"HoldCool": 1.0,
+			"HeatHold": 1.0,
+			"HeatCool": 1.0,
+			"CoolHold": 1.0,
+			"CoolHeat": 1.0
+		}
+	},
+	"Heater": {
+		"type": "sim"
+	},
+	"Stirrer": {
+		"type": "sim"
+	},
+	"TempSensor": {
+		"type": "sim",
+		"temp_offset": 0.0,
+		"sigma": 0.05,
+		"stirrer_sigma": 0.0,
+		"stirrer_tau": 20.0
+	},
+	"Pot": {
+		"mass": 5.96,
+		"material": "Edelstahl 18/10",
+		"L": 0.2,
+		"Td": 17,
+		"water_mass": 22,
+		"volumen": 30
+	}
+}
diff --git a/tests/utils/fixtures/missing_outer_kp.json b/tests/utils/fixtures/missing_outer_kp.json
new file mode 100644
index 0000000..0fbe063
--- /dev/null
+++ b/tests/utils/fixtures/missing_outer_kp.json
@@ -0,0 +1,64 @@
+{
+	"ambient_temperature": 20,
+	"log_interval": 300,
+	"TempCtrl": {
+		"pid_type": "Smith",
+		"beta": 0.05,
+		"Outer": {
+			"ki": 0.0,
+			"kd": 0.0,
+			"kt": 0.0,
+			"y_hold_min": -0.1
+		},
+		"Inner": {
+			"Heat": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5
+			},
+			"Hold": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5,
+				"yi_max": 0.3
+			},
+			"Cool": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5
+			}
+		},
+		"Thresholds": {
+			"HoldHeat": 1.0,
+			"HoldCool": 1.0,
+			"HeatHold": 1.0,
+			"HeatCool": 1.0,
+			"CoolHold": 1.0,
+			"CoolHeat": 1.0
+		}
+	},
+	"Heater": {
+		"type": "sim"
+	},
+	"Stirrer": {
+		"type": "sim"
+	},
+	"TempSensor": {
+		"type": "sim",
+		"temp_offset": 0.0,
+		"sigma": 0.05,
+		"stirrer_sigma": 0.0,
+		"stirrer_tau": 20.0
+	},
+	"Pot": {
+		"mass": 5.96,
+		"material": "Edelstahl 18/10",
+		"L": 0.2,
+		"Td": 17,
+		"water_mass": 22,
+		"volumen": 30
+	}
+}
diff --git a/tests/utils/fixtures/missing_pid_type.json b/tests/utils/fixtures/missing_pid_type.json
new file mode 100644
index 0000000..57c4c8f
--- /dev/null
+++ b/tests/utils/fixtures/missing_pid_type.json
@@ -0,0 +1,64 @@
+{
+	"ambient_temperature": 20,
+	"log_interval": 300,
+	"TempCtrl": {
+		"beta": 0.05,
+		"Outer": {
+			"kp": 0.4,
+			"ki": 0.0,
+			"kd": 0.0,
+			"kt": 0.0,
+			"y_hold_min": -0.1
+		},
+		"Inner": {
+			"Heat": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5
+			},
+			"Hold": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5,
+				"yi_max": 0.3
+			},
+			"Cool": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5
+			}
+		},
+		"Thresholds": {
+			"HoldHeat": 1.0,
+			"HoldCool": 1.0,
+			"HeatHold": 1.0,
+			"HeatCool": 1.0,
+			"CoolHold": 1.0,
+			"CoolHeat": 1.0
+		}
+	},
+	"Heater": {
+		"type": "sim"
+	},
+	"Stirrer": {
+		"type": "sim"
+	},
+	"TempSensor": {
+		"type": "sim",
+		"temp_offset": 0.0,
+		"sigma": 0.05,
+		"stirrer_sigma": 0.0,
+		"stirrer_tau": 20.0
+	},
+	"Pot": {
+		"mass": 5.96,
+		"material": "Edelstahl 18/10",
+		"L": 0.2,
+		"Td": 17,
+		"water_mass": 22,
+		"volumen": 30
+	}
+}
diff --git a/tests/utils/fixtures/missing_tempctrl_section.json b/tests/utils/fixtures/missing_tempctrl_section.json
new file mode 100644
index 0000000..68cb7d8
--- /dev/null
+++ b/tests/utils/fixtures/missing_tempctrl_section.json
@@ -0,0 +1,25 @@
+{
+	"ambient_temperature": 20,
+	"log_interval": 300,
+	"Heater": {
+		"type": "sim"
+	},
+	"Stirrer": {
+		"type": "sim"
+	},
+	"TempSensor": {
+		"type": "sim",
+		"temp_offset": 0.0,
+		"sigma": 0.05,
+		"stirrer_sigma": 0.0,
+		"stirrer_tau": 20.0
+	},
+	"Pot": {
+		"mass": 5.96,
+		"material": "Edelstahl 18/10",
+		"L": 0.2,
+		"Td": 17,
+		"water_mass": 22,
+		"volumen": 30
+	}
+}
diff --git a/tests/utils/fixtures/valid_normal.json b/tests/utils/fixtures/valid_normal.json
new file mode 100644
index 0000000..dd11637
--- /dev/null
+++ b/tests/utils/fixtures/valid_normal.json
@@ -0,0 +1,65 @@
+{
+	"ambient_temperature": 20,
+	"log_interval": 300,
+	"TempCtrl": {
+		"pid_type": "Normal",
+		"beta": 0.05,
+		"Outer": {
+			"kp": 0.4,
+			"ki": 0.0,
+			"kd": 0.0,
+			"kt": 0.0,
+			"y_hold_min": -0.1
+		},
+		"Inner": {
+			"Heat": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5
+			},
+			"Hold": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5,
+				"yi_max": 0.3
+			},
+			"Cool": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5
+			}
+		},
+		"Thresholds": {
+			"HoldHeat": 1.0,
+			"HoldCool": 1.0,
+			"HeatHold": 1.0,
+			"HeatCool": 1.0,
+			"CoolHold": 1.0,
+			"CoolHeat": 1.0
+		}
+	},
+	"Heater": {
+		"type": "sim"
+	},
+	"Stirrer": {
+		"type": "sim"
+	},
+	"TempSensor": {
+		"type": "sim",
+		"temp_offset": 0.0,
+		"sigma": 0.05,
+		"stirrer_sigma": 0.0,
+		"stirrer_tau": 20.0
+	},
+	"Pot": {
+		"mass": 5.96,
+		"material": "Edelstahl 18/10",
+		"L": 0.2,
+		"Td": 17,
+		"water_mass": 22,
+		"volumen": 30
+	}
+}
diff --git a/tests/utils/fixtures/valid_real.json b/tests/utils/fixtures/valid_real.json
new file mode 100644
index 0000000..f615ec5
--- /dev/null
+++ b/tests/utils/fixtures/valid_real.json
@@ -0,0 +1,66 @@
+{
+	"ambient_temperature": 20,
+	"log_interval": 300,
+	"TempCtrl": {
+		"pid_type": "Smith",
+		"beta": 0.05,
+		"Outer": {
+			"kp": 0.4,
+			"ki": 0.0,
+			"kd": 0.0,
+			"kt": 0.0,
+			"y_hold_min": -0.1
+		},
+		"Inner": {
+			"Heat": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5
+			},
+			"Hold": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5,
+				"yi_max": 0.3
+			},
+			"Cool": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5
+			}
+		},
+		"Thresholds": {
+			"HoldHeat": 1.0,
+			"HoldCool": 1.0,
+			"HeatHold": 1.0,
+			"HeatCool": 1.0,
+			"CoolHold": 1.0,
+			"CoolHeat": 1.0
+		}
+	},
+	"Heater": {
+		"type": "hendi",
+		"port": "/dev/ttyUSB0",
+		"speed": "115200"
+	},
+	"Stirrer": {
+		"type": "pololu1376",
+		"port": "/dev/ttyACM0",
+		"speed": "115200"
+	},
+	"TempSensor": {
+		"type": "max31865",
+		"temp_offset": -0.15
+	},
+	"Pot": {
+		"mass": 5.96,
+		"material": "Edelstahl 18/10",
+		"L": 0.2,
+		"Td": 17,
+		"water_mass": 22,
+		"volumen": 30
+	}
+}
diff --git a/tests/utils/fixtures/valid_sim.json b/tests/utils/fixtures/valid_sim.json
new file mode 100644
index 0000000..37341fc
--- /dev/null
+++ b/tests/utils/fixtures/valid_sim.json
@@ -0,0 +1,65 @@
+{
+	"ambient_temperature": 20,
+	"log_interval": 300,
+	"TempCtrl": {
+		"pid_type": "Smith",
+		"beta": 0.05,
+		"Outer": {
+			"kp": 0.4,
+			"ki": 0.0,
+			"kd": 0.0,
+			"kt": 0.0,
+			"y_hold_min": -0.1
+		},
+		"Inner": {
+			"Heat": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5
+			},
+			"Hold": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5,
+				"yi_max": 0.3
+			},
+			"Cool": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5
+			}
+		},
+		"Thresholds": {
+			"HoldHeat": 1.0,
+			"HoldCool": 1.0,
+			"HeatHold": 1.0,
+			"HeatCool": 1.0,
+			"CoolHold": 1.0,
+			"CoolHeat": 1.0
+		}
+	},
+	"Heater": {
+		"type": "sim"
+	},
+	"Stirrer": {
+		"type": "sim"
+	},
+	"TempSensor": {
+		"type": "sim",
+		"temp_offset": 0.0,
+		"sigma": 0.05,
+		"stirrer_sigma": 0.0,
+		"stirrer_tau": 20.0
+	},
+	"Pot": {
+		"mass": 5.96,
+		"material": "Edelstahl 18/10",
+		"L": 0.2,
+		"Td": 17,
+		"water_mass": 22,
+		"volumen": 30
+	}
+}
diff --git a/tests/utils/fixtures/wrong_type_kp_string.json b/tests/utils/fixtures/wrong_type_kp_string.json
new file mode 100644
index 0000000..9d45527
--- /dev/null
+++ b/tests/utils/fixtures/wrong_type_kp_string.json
@@ -0,0 +1,65 @@
+{
+	"ambient_temperature": 20,
+	"log_interval": 300,
+	"TempCtrl": {
+		"pid_type": "Smith",
+		"beta": 0.05,
+		"Outer": {
+			"kp": 0.4,
+			"ki": 0.0,
+			"kd": 0.0,
+			"kt": 0.0,
+			"y_hold_min": -0.1
+		},
+		"Inner": {
+			"Heat": {
+				"kp": "0.08",
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5
+			},
+			"Hold": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5,
+				"yi_max": 0.3
+			},
+			"Cool": {
+				"kp": 0.08,
+				"ki": 0.02,
+				"kd": 0.0,
+				"kt": 1.5
+			}
+		},
+		"Thresholds": {
+			"HoldHeat": 1.0,
+			"HoldCool": 1.0,
+			"HeatHold": 1.0,
+			"HeatCool": 1.0,
+			"CoolHold": 1.0,
+			"CoolHeat": 1.0
+		}
+	},
+	"Heater": {
+		"type": "sim"
+	},
+	"Stirrer": {
+		"type": "sim"
+	},
+	"TempSensor": {
+		"type": "sim",
+		"temp_offset": 0.0,
+		"sigma": 0.05,
+		"stirrer_sigma": 0.0,
+		"stirrer_tau": 20.0
+	},
+	"Pot": {
+		"mass": 5.96,
+		"material": "Edelstahl 18/10",
+		"L": 0.2,
+		"Td": 17,
+		"water_mass": 22,
+		"volumen": 30
+	}
+}
diff --git a/tests/utils/fixtures/wrong_type_tempctrl_not_dict.json b/tests/utils/fixtures/wrong_type_tempctrl_not_dict.json
new file mode 100644
index 0000000..a267602
--- /dev/null
+++ b/tests/utils/fixtures/wrong_type_tempctrl_not_dict.json
@@ -0,0 +1,26 @@
+{
+	"ambient_temperature": 20,
+	"log_interval": 300,
+	"TempCtrl": "Smith",
+	"Heater": {
+		"type": "sim"
+	},
+	"Stirrer": {
+		"type": "sim"
+	},
+	"TempSensor": {
+		"type": "sim",
+		"temp_offset": 0.0,
+		"sigma": 0.05,
+		"stirrer_sigma": 0.0,
+		"stirrer_tau": 20.0
+	},
+	"Pot": {
+		"mass": 5.96,
+		"material": "Edelstahl 18/10",
+		"L": 0.2,
+		"Td": 17,
+		"water_mass": 22,
+		"volumen": 30
+	}
+}
diff --git a/tests/utils/test_config_fixtures.py b/tests/utils/test_config_fixtures.py
new file mode 100644
index 0000000..73cad2d
--- /dev/null
+++ b/tests/utils/test_config_fixtures.py
@@ -0,0 +1,47 @@
+import json
+import os
+import unittest
+
+from utils.config_validate import ConfigError, validate_temp_ctrl
+
+FIXTURES_DIR = os.path.join(os.path.dirname(__file__), 'fixtures')
+
+# filename -> None if the fixture should pass validation, otherwise a regex
+# matched against the raised ConfigError's message.
+CASES = {
+    'valid_sim.json': None,
+    'valid_real.json': None,
+    'valid_normal.json': None,
+    'legacy_unknown_keys.json': None,
+    'missing_outer_kp.json': r'TempCtrl\.Outer\.kp',
+    'missing_inner_hold_branch.json': r'TempCtrl\.Inner\.Hold',
+    'missing_pid_type.json': r'TempCtrl\.pid_type',
+    'missing_tempctrl_section.json': r'missing required config key: TempCtrl$',
+    'wrong_type_kp_string.json': r'TempCtrl\.Inner\.Heat\.kp',
+    'wrong_type_tempctrl_not_dict.json': r'TempCtrl must be dict',
+}
+
+
+def _load(name):
+    with open(os.path.join(FIXTURES_DIR, name)) as f:
+        return json.load(f)
+
+
+class TestConfigFixtures(unittest.TestCase):
+    def test_every_fixture_file_is_covered(self):
+        on_disk = {f for f in os.listdir(FIXTURES_DIR) if f.endswith('.json')}
+        self.assertEqual(on_disk, set(CASES), "fixtures/ and CASES have drifted apart")
+
+    def test_fixtures_match_expected_outcome(self):
+        for name, expected_error in CASES.items():
+            with self.subTest(fixture=name):
+                config = _load(name)
+                if expected_error is None:
+                    validate_temp_ctrl(config)  # must not raise
+                else:
+                    with self.assertRaisesRegex(ConfigError, expected_error):
+                        validate_temp_ctrl(config)
+
+
+if __name__ == '__main__':
+    unittest.main()