diff --git a/tests/conftest.py b/tests/conftest.py
index 970f99e..b1dc018 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -37,10 +37,9 @@
 TRIANGLE_FILES = FIXTURES_PDDL_FILES / "triangle-tireworld"
 BLOCKSWORLD_FOND_FILES = FIXTURES_PDDL_FILES / "blocksworld_fond"
 
-# TODO once missing features are supported, uncomment this
-# DOMAIN_FILES = [
-#     *FIXTURES_PDDL_FILES.glob("./**/domain.pddl")
-# ]
+DOMAIN_FILES = [
+    *FIXTURES_PDDL_FILES.glob("./**/domain.pddl")
+]
 
 DOMAIN_NAMES = [
     "acrobatics",
@@ -48,10 +47,10 @@
     "blocksworld-ipc08",
     "blocksworld_fond",
     "doors",
-    # "earth_observation",
+    "earth_observation",
     "elevators",
     # "faults-ipc08",
-    # "first-responders-ipc08",
+    "first-responders-ipc08",
     "islands",
     "maintenance-sequential-satisficing-ipc2014",
     "miner",
@@ -65,10 +64,6 @@
     "hello-world-functions",
 ]
 
-DOMAIN_FILES = [
-    FIXTURES_PDDL_FILES / domain_name / "domain.pddl" for domain_name in DOMAIN_NAMES
-]
-
 PROBLEM_FILES = list(
     itertools.chain(
         *[
diff --git a/tests/test_function.py b/tests/test_function.py
new file mode 100644
index 0000000..56f0937
--- /dev/null
+++ b/tests/test_function.py
@@ -0,0 +1,42 @@
+#
+# Copyright 2021-2023 WhiteMech
+#
+# ------------------------------
+#
+# This file is part of pddl.
+#
+# Use of this source code is governed by an MIT-style
+# license that can be found in the LICENSE file or at
+# https://opensource.org/licenses/MIT.
+#
+"""This module contains tests for a PDDL function."""
+
+from pddl.logic import variables
+from pddl.logic.functions import Function
+
+
+class TestFunctionEmpty:
+    """Test the empty function."""
+
+    def setup_method(self):
+        """Set up the tests."""
+        self.function = Function("empty_function")
+
+    def test_name(self):
+        """Test the name getter."""
+        assert self.function.name == "empty_function"
+
+    def test_terms(self):
+        """Test the parameters getter."""
+        assert self.function.terms == ()
+
+    def test_arity(self):
+        """Test the arity getter."""
+        assert self.function.arity == 0
+
+
+def test_build_simple_function():
+    """Test a simple PDDL action."""
+    x, y, z = variables("x y z", types=["type1"])
+    function = Function("simple_function", x, y, z)
+    assert function
diff --git a/tests/test_function_operators.py b/tests/test_function_operators.py
new file mode 100644
index 0000000..0503119
--- /dev/null
+++ b/tests/test_function_operators.py
@@ -0,0 +1,60 @@
+#
+# Copyright 2021-2023 WhiteMech
+#
+# ------------------------------
+#
+# This file is part of pddl.
+#
+# Use of this source code is governed by an MIT-style
+# license that can be found in the LICENSE file or at
+# https://opensource.org/licenses/MIT.
+#
+"""This module contains tests for PDDL function operators."""
+from pddl.parser.symbols import Symbols
+
+from pddl.logic import variables
+from pddl.logic.functions import Function, FunctionOperator
+
+
+class TestFunctionOperators:
+    """Test the function operators."""
+
+    def setup_method(self):
+        """Set up the tests."""
+        x, y = variables("x y", types=["type1"])
+        z = variables("z", types=["type2"])
+        self.function = Function("function_1")
+        self.function_op = FunctionOperator(
+            self.function,
+            3,
+            Symbols.EQUAL
+        )
+
+    def test_function(self):
+        """Test the function getter."""
+        assert self.function_op.function == self.function
+
+    def test_symbol(self):
+        """Test the symbol getter."""
+        assert self.function_op.symbol == Symbols.EQUAL.value
+
+    def test_value(self):
+        """Test the value getter."""
+        assert self.function_op.value == 3
+
+    def test_equal(self):
+        """Test the equal operator."""
+        other = FunctionOperator(
+            self.function,
+            3,
+            Symbols.EQUAL.value
+        )
+        assert self.function_op == other
+
+    def test_str(self):
+        """Test the str operator."""
+        assert str(self.function_op) == f"({self.function_op.symbol.value} {self.function} {self.function_op.value})"
+
+    def test_repr(self):
+        """Test the repr operator."""
+        assert repr(self.function_op) == f"FunctionOperator({self.function}, {self.function_op.value})"
diff --git a/tests/test_parser/test_domain.py b/tests/test_parser/test_domain.py
index c193b65..b49dd88 100644
--- a/tests/test_parser/test_domain.py
+++ b/tests/test_parser/test_domain.py
@@ -293,3 +293,27 @@ def test_variables_repetition_allowed_if_same_type() -> None:
     """
     )
     DomainParser()(domain_str)
+
+
+def test_check_action_costs_requirement_with_total_cost() -> None:
+    """Check action costs requirement when total-cost is specified."""
+    domain_str = dedent(
+        """
+    (define (domain test)
+        (:requirements :typing)
+        (:types t1 t2)
+        (:predicates (p ?x - t1 ?y - t2))
+        (:functions (total-cost))
+        (:action a
+            :parameters (?x - t1 ?y - t2)
+            :precondition (and (p ?x ?x))
+            :effect (p ?x ?x)
+        )
+    )
+    """
+    )
+    with pytest.raises(
+        lark.exceptions.VisitError,
+        match=r"action costs requirement is not specified, but the total-cost function is specified.",
+    ):
+        DomainParser()(domain_str)