diff --git a/src/PreferredSearchCoreService.php b/src/PreferredSearchCoreService.php
index 2c7fd3b..84f213a 100644
--- a/src/PreferredSearchCoreService.php
+++ b/src/PreferredSearchCoreService.php
@@ -248,6 +248,9 @@ class PreferredSearchCoreService {
 
       $possible_core_ids[] = $this->acquiaIdentifier . '.' . $ah_env . '.' . $sites_foldername;
 
+      // Add eligible fallback core names for Acquia Site Factory sites.
+      $this->addACSFFallbackCores($possible_core_ids, $ah_env, $sites_foldername);
+
     }
 
     // For production-only, we allow auto-connecting to the suffix-less core
@@ -263,4 +266,29 @@ class PreferredSearchCoreService {
 
   }
 
+  /**
+   * Add list of eligible fallback cores when site is within Site Factory.
+   *
+   * @param $possible_core_ids
+   * @param $ah_env
+   * @param $sites_foldername
+   *
+   * For Site Factory sites, these core names are acceptable:
+   *   [id].[env].default
+   *   [id]  (only if environment is 01live)
+   */
+  protected function addACSFFallbackCores(&$possible_core_ids, $ah_env, $sites_foldername) {
+    $realm = !empty($_ENV['AH_REALM']) ? $_ENV['AH_REALM'] : '';
+    if ($realm == 'enterprise-g1' || preg_match('/gardens$/', $realm)) {
+      // No need to add 'default' again if we already set it.
+      if ($sites_foldername != 'default') {
+        $possible_core_ids[] = $this->acquiaIdentifier . '.' . $ah_env . '.default';
+      }
+      // For environments that match [num]live (01live, 02live, etc.)
+      if (preg_match('/^[\d]+live$/', $ah_env)) {
+           $possible_core_ids[] = $this->acquiaIdentifier;
+      }
+    }
+  }
+
 }
diff --git a/tests/modules/acquia_search_test/src/EventSubscriber/AcquiaSearchTestSubscriber.php b/tests/modules/acquia_search_test/src/EventSubscriber/AcquiaSearchTestSubscriber.php
index 18d042e..548194c 100644
--- a/tests/modules/acquia_search_test/src/EventSubscriber/AcquiaSearchTestSubscriber.php
+++ b/tests/modules/acquia_search_test/src/EventSubscriber/AcquiaSearchTestSubscriber.php
@@ -24,6 +24,7 @@ class AcquiaSearchTestSubscriber implements EventSubscriberInterface {
         'AH_SITE_NAME',
         'AH_SITE_GROUP',
         'AH_PRODUCTION',
+        'AH_REALM',
       ];
       foreach ($allowed_keys as $key) {
         $value = $event->getRequest()->query->get($key);
diff --git a/tests/src/Functional/AcquiaConnectorSearchOverrideTest.php b/tests/src/Functional/AcquiaConnectorSearchOverrideTest.php
index 2547761..d283ae1 100644
--- a/tests/src/Functional/AcquiaConnectorSearchOverrideTest.php
+++ b/tests/src/Functional/AcquiaConnectorSearchOverrideTest.php
@@ -108,6 +108,8 @@ class AcquiaConnectorSearchOverrideTest extends BrowserTestBase {
     $this->caseAcquiaHostingEnvironmentDetectedNoAvailableCores();
     $this->caseAcquiaHostingProdEnvironmentDetectedWithoutProdFlag();
     $this->caseAcquiaHostingProdEnvironmentDetectedWithProdFlag();
+    $this->caseACSFProdEnvironmentDetectedWithProdFlag();
+    $this->caseACSFEnvironmentDetectedNoAvailableCores();
 
   }
 
@@ -184,6 +186,7 @@ class AcquiaConnectorSearchOverrideTest extends BrowserTestBase {
     $this->assertText('The following Acquia Search Solr index IDs would have worked for your current environment');
     $this->assertText($this->id . '.test.' . $this->getDbName());
     $this->assertText($this->id . '.test.' . $this->getSiteFolderName());
+    $this->assertNoText($this->id . '.test.default');
 
     //phpcs:disable
     //$delete_btn = $this->xpath('//input[@value="Delete all indexed data on this server"]');
@@ -218,6 +221,7 @@ class AcquiaConnectorSearchOverrideTest extends BrowserTestBase {
     $this->assertText('The following Acquia Search Solr index IDs would have worked for your current environment');
     $this->assertText($this->id . '.prod.' . $this->getDbName());
     $this->assertText($this->id . '.prod.' . $this->getSiteFolderName());
+    $this->assertNoText($this->id . '.prod.default');
 
     //phpcs:disable
     //$delete_btn = $this->xpath('//input[@value="Delete all indexed data on this server"]');
@@ -262,6 +266,40 @@ class AcquiaConnectorSearchOverrideTest extends BrowserTestBase {
 
   }
 
+  /**
+   * Acquia ACSF test environment. According to the mock, no cores
+   * available for the Test environment so it is read only. Also, an eligible
+   * corename will be [id].[env].default
+   */
+  public function caseACSFEnvironmentDetectedNoAvailableCores() {
+
+    $overrides = [
+      'env-overrides' => 1,
+      'AH_SITE_ENVIRONMENT' => '99test',
+      'AH_SITE_NAME' => 'testsite199test',
+      'AH_SITE_GROUP' => 'testsite1',
+      'AH_REALM' => 'enterprise-g1',
+    ];
+
+    $this->drupalGet('/admin/config/search/search-api/server/' . $this->server, ['query' => $overrides ]);
+
+    $this->assertText('automatically enforced read-only mode on this connection.');
+
+    $this->assertText('The following Acquia Search Solr index IDs would have worked for your current environment');
+    $this->assertText($this->id . '.99test.' . $this->_getDbName());
+    $this->assertText($this->id . '.99test.' . $this->_getSiteFolderName());
+    $this->assertText($this->id . '.99test.default');
+
+    $delete_btn = $this->xpath('//input[@value="Delete all indexed data on this server"]');
+    $this->assertEqual((string) $delete_btn[0]['disabled'], 'disabled');
+
+    $this->drupalGet('/admin/config/search/search-api/index/' . $this->index, ['query' => $overrides ]);
+
+    // On index edit page, check the read-only mode state.
+    $this->assertText('automatically enforced read-only mode on this connection.');
+
+  }
+
   /**
    * Connect to the Acquia Subscription.
    */