diff --git a/GeoHealthCheck/app.py b/GeoHealthCheck/app.py
index ff317bca..504e5f13 100644
--- a/GeoHealthCheck/app.py
+++ b/GeoHealthCheck/app.py
@@ -767,7 +767,10 @@ def edit_resource(resource_identifier):
         flash(gettext('Resource not found'), 'danger')
         return redirect(request.referrer)
 
-    probes_avail = views.get_probes_avail(resource.resource_type, resource)
+    # Editing must remain possible when the monitored service is unavailable.
+    # Probe expansion can make live GetCapabilities requests.
+    probes_avail = views.get_probes_avail(resource.resource_type, resource,
+                                         expand_params=False)
 
     suggestions = json.dumps(Recipient.get_suggestions('email',
                                                        g.user.username))
diff --git a/GeoHealthCheck/probe.py b/GeoHealthCheck/probe.py
index 937e0866..5ec03d23 100644
--- a/GeoHealthCheck/probe.py
+++ b/GeoHealthCheck/probe.py
@@ -90,6 +90,9 @@ class Probe(Plugin):
 
     def __init__(self):
         Plugin.__init__(self)
+        # expand_params() modifies these definitions for a Resource. Keep
+        # choices from one Resource out of other requests and edit forms.
+        self.PARAM_DEFS = Plugin.copy(self.PARAM_DEFS)
         self._resource = None
         self._session = create_requests_retry_session()
 
diff --git a/GeoHealthCheck/templates/includes/probe_edit_form.html b/GeoHealthCheck/templates/includes/probe_edit_form.html
index 00198f94..53e88df9 100644
--- a/GeoHealthCheck/templates/includes/probe_edit_form.html
+++ b/GeoHealthCheck/templates/includes/probe_edit_form.html
@@ -28,8 +28,8 @@
                                         {% set probe_param_type = 'list'  %}
                                         {% set probe_param_val = probe_param_val|join(',') %}
                                      {% endif %}
-                                     <!-- parameters: use select box when range defined, else regular input box -->
-                                     {% if probe_param_range %}
+                                     <!-- Keep a saved value editable when it is absent from static choices. -->
+                                     {% if probe_param_range and (not probe_param_val or probe_param_val in probe_param_range) %}
                                      <select class="form-control" data-probe-param="{{ param }}" data-probe-param-type="{{ probe_param_type }}">
                                          <!-- select box for range: select value if defined else use default value if defined -->
                                          {% for probe_param_range_val in probe_param_range %}
diff --git a/GeoHealthCheck/views.py b/GeoHealthCheck/views.py
index 4d4c8b7e..320d9e88 100644
--- a/GeoHealthCheck/views.py
+++ b/GeoHealthCheck/views.py
@@ -207,11 +207,12 @@ def get_query_field_term(query):
     return [field, term]
 
 
-def get_probes_avail(resource_type=None, resource=None):
+def get_probes_avail(resource_type=None, resource=None, expand_params=True):
     """
     Get all available Probes with their attributes.
     :param resource_type: optional resource type e.g. OGC:WMS
     :param resource: optional Resource instance
+    :param expand_params: fetch resource metadata to populate parameter choices
     :return:
     """
 
@@ -227,7 +228,7 @@ def get_probes_avail(resource_type=None, resource=None):
     for probe_class in probe_classes:
         probe = Factory.create_obj(probe_class)
         if probe:
-            if resource:
+            if resource and expand_params:
                 try:
                     probe._resource = resource
                     probe.expand_params(resource)
diff --git a/tests/run_tests.py b/tests/run_tests.py
index 067960a6..cc5c14f7 100644
--- a/tests/run_tests.py
+++ b/tests/run_tests.py
@@ -41,5 +41,8 @@
 # Shorthand to run all test scripts in tests dir.
 # TODO use nose or more intelligent test_*.py discovery
 if __name__ == '__main__':
-    unittest.main(module='test_plugins', exit=False)
-    unittest.main(module='test_resources')
+    results = [
+        unittest.main(module=module, exit=False).result
+        for module in ('test_plugins', 'test_resources', 'test_edit_offline')
+    ]
+    sys.exit(0 if all(result.wasSuccessful() for result in results) else 1)
diff --git a/tests/test_edit_offline.py b/tests/test_edit_offline.py
new file mode 100644
index 00000000..cd59a831
--- /dev/null
+++ b/tests/test_edit_offline.py
@@ -0,0 +1,70 @@
+"""The resource editor must work while an OGC service is unavailable."""
+
+import os
+import unittest
+from unittest.mock import patch
+
+from app import APP
+from models import DB, CheckVars, ProbeVars, Resource, User, load_data
+from GeoHealthCheck.plugins.probe.wmts import WmtsGetTile, WmtsGetTileAll
+
+
+TEST_DIR = os.path.dirname(os.path.abspath(__file__))
+
+
+class OfflineResourceEditTest(unittest.TestCase):
+    def setUp(self):
+        with patch('models.util.geocode', return_value=(0.0, 0.0)):
+            load_data('%s/data/minimal.json' % TEST_DIR)
+            owner = User.query.filter_by(username='admin').one()
+            resource = Resource(owner, 'OGC:WMTS', 'Offline WMTS',
+                                'https://example.invalid/wmts', [])
+            resource.active = False
+
+        probe = ProbeVars(resource,
+                          'GeoHealthCheck.plugins.probe.wmts.WmtsGetTile',
+                          {'layers': ['layer_a', 'layer_b'],
+                           'tilematrixset': 'EPSG:4326',
+                           'kvprest': 'KVP'})
+        CheckVars(probe,
+                  'GeoHealthCheck.plugins.check.checks.HttpStatusNoError',
+                  {})
+        DB.session.add(resource)
+        DB.session.commit()
+        self.resource_id = resource.identifier
+        self.client = APP.test_client()
+        response = self.client.post('/login',
+                                    data={'username': 'admin',
+                                          'password': 'admin'})
+        self.assertEqual(response.status_code, 302)
+
+    def tearDown(self):
+        DB.session.remove()
+        DB.drop_all()
+
+    def test_edit_page_does_not_expand_remote_probe_parameters(self):
+        with patch.object(WmtsGetTile, 'expand_params') as expand_tile, \
+                patch.object(WmtsGetTileAll, 'expand_params') as expand_all:
+            response = self.client.get(
+                '/resource/%d/edit' % self.resource_id)
+
+        self.assertEqual(response.status_code, 200)
+        expand_tile.assert_not_called()
+        expand_all.assert_not_called()
+        self.assertIn(b'value="layer_a,layer_b"', response.data)
+        # A saved value outside the static choices must not become the
+        # browser's first choice when the form is saved.
+        self.assertIn(b'value="EPSG:4326"', response.data)
+        self.assertIn(b'value="KVP"', response.data)
+
+    def test_probe_parameter_definitions_are_isolated(self):
+        first = WmtsGetTile()
+        second = WmtsGetTile()
+        first.PARAM_DEFS['layers']['range'] = ['another_resource']
+
+        self.assertIsNone(second.PARAM_DEFS['layers']['range'])
+        self.assertIsNone(WmtsGetTile.PARAM_DEFS['layers']['range'])
+
+
+if __name__ == '__main__':
+    unittest.main()