diff --git a/companion/src/firmwares/boardjson.cpp b/companion/src/firmwares/boardjson.cpp
index ba43752fd..494771305 100644
--- a/companion/src/firmwares/boardjson.cpp
+++ b/companion/src/firmwares/boardjson.cpp
@@ -135,8 +135,10 @@ void BoardJson::afterLoadFixups(Board::Type board, InputsTable * inputs, Switche
   }
 
   //  Flex switches are not listed in json file for these radios
-  int count = IS_RADIOMASTER_TX16S(board) || IS_RADIOMASTER_MT12(board) ||
-              IS_HELLORADIOSKY_V16(board) ? 2 : 0;
+  //  Must match the targets that set FLEXSW (MAX_FLEX_SWITCHES) in the firmware
+  int count = IS_RADIOMASTER_TX16S(board) || IS_RADIOMASTER_TX16SMK3(board) ||
+              IS_FATFISH_F16(board) || IS_HELLORADIOSKY_V16(board) ||
+              IS_RADIOMASTER_MT12(board) ? 2 : 0;
 
   for (int i = 1; i <= count; i++) {
     QString tag = QString("FL%1").arg(i);
diff --git a/companion/src/tests/flex_switches_test.cpp b/companion/src/tests/flex_switches_test.cpp
new file mode 100644
index 000000000..128e1167d
--- /dev/null
+++ b/companion/src/tests/flex_switches_test.cpp
@@ -0,0 +1,81 @@
+/*
+ * Copyright (C) EdgeTX
+ *
+ * Based on code named
+ *   opentx - https://github.com/opentx/opentx
+ *   th9x - http://code.google.com/p/th9x
+ *   er9x - http://code.google.com/p/er9x
+ *   gruvin9x - http://code.google.com/p/gruvin9x
+ *
+ * License GPLv2: http://www.gnu.org/licenses/gpl-2.0.html
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+// Flex switches (FL1/FL2) are not listed in the hwdefs json, so Companion adds
+// them itself (BoardJson::afterLoadFixups). The set of boards must match the
+// firmware targets that define FLEXSW (MAX_FLEX_SWITCHES); a board missing
+// here loses FL1/FL2 and everything indexed after them.
+
+#include "gtests.h"
+
+#include "firmwares/eeprominterface.h"
+#include "firmwares/boardjson.h"
+#include "firmwares/boards.h"
+
+namespace {
+
+struct FlexCase {
+  const char* flavour;
+  int stdSwitches;  // switches in the hwdefs json, including function switches
+};
+
+class FlexSwitchesBoard : public ::testing::TestWithParam<FlexCase>
+{
+ protected:
+  void SetUp() override
+  {
+    Firmware::setCurrentVariant(Firmware::getFirmwareForFlavour(GetParam().flavour));
+    ASSERT_NE(getCurrentFirmware(), nullptr)
+        << GetParam().flavour << " firmware must be registered for this test";
+  }
+};
+
+}  // namespace
+
+// Firmware indexes flex switches straight after the physical switches
+// (switchGetMaxSwitches()), which is what the simulator and YAML rely on.
+TEST_P(FlexSwitchesBoard, FlexSwitchesFollowPhysicalSwitches)
+{
+  const Board::Type board = getCurrentBoard();
+  const int n = GetParam().stdSwitches;
+
+  EXPECT_EQ(Boards::getCapability(board, Board::FlexSwitches), 2);
+  EXPECT_EQ(Boards::getSwitchYamlIndex(QStringLiteral("FL1"), BoardJson::YLT_REF, board), n);
+  EXPECT_EQ(Boards::getSwitchYamlIndex(QStringLiteral("FL2"), BoardJson::YLT_REF, board), n + 1);
+}
+
+INSTANTIATE_TEST_SUITE_P(
+    FlexBoards, FlexSwitchesBoard,
+    ::testing::Values(FlexCase{"tx16s", 10}, FlexCase{"tx16smk3", 16},
+                      FlexCase{"f16", 8}, FlexCase{"v16", 10}, FlexCase{"mt12", 4}),
+    [](const ::testing::TestParamInfo<FlexCase>& i) { return std::string(i.param.flavour); });
+
+// Radios whose firmware has MAX_FLEX_SWITCHES == 0 (no FLEXSW) must not gain
+// any, even though most of them have flex inputs.
+TEST(FlexSwitchesAbsent, BoardsWithoutFlexSwitches)
+{
+  for (const char* flavour : {"x10", "t16", "t18", "st16", "pl18"}) {
+    Firmware* fw = Firmware::getFirmwareForFlavour(flavour);
+    ASSERT_NE(fw, nullptr) << flavour << " firmware must be registered for this test";
+    Firmware::setCurrentVariant(fw);
+    EXPECT_EQ(Boards::getCapability(getCurrentBoard(), Board::FlexSwitches), 0) << flavour;
+  }
+}