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Troubleshooting

Use this guide to resolve common problems with firmware installation, setup, and everyday operation.

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The status light is on or blinking

With HomeKit-compatible and Matter-compatible firmware, an unlit RGB LED normally means the controller is operating. Solid red indicates that the indoor unit is not responding over CN105; a yellow pulse indicates safe mode. Use the status lights reference to identify other colors and patterns. ESPHome exposes the LED as a Home Assistant light entity; those status meanings do not apply.

What the light does while you hold the button

The light shows which button action is selected. Serin Link pairing actions happen when you release the button, but holding it for 10 seconds resets Wi-Fi immediately, while the button is still pressed. Check the button hold reference before holding it: some actions remove pairings or Wi-Fi credentials. For a planned Wi-Fi change or reset, follow the maintenance guide.

The light is pulsing yellow
  • After repeated crashes, the controller starts in safe mode. Wi-Fi and the web interface remain available, and a banner indicates that safe mode is active.
  • Open the web interface and update the firmware, or install the firmware over USB from the flash page.
  • If the controller returns to safe mode after a USB installation, the cause is more likely to be wiring or power than firmware. Check the CN105 connection and contact support.

Flashing and USB

Controller is not detected over USB, or no serial port appears
  • Use a USB data cable, not a charge-only cable. Test the cable by transferring files to a phone or another device.
  • The NanoC6 and Atom S3 Lite need no driver. A DIY board with a CP210x or CH340 USB chip may need that chip's driver, such as the SiLabs CP210x driver.
  • Try a different USB port, preferably directly on the computer, not through a hub.
  • Ensure you are using Chrome, Edge, or Brave. Web Serial is not supported in Safari or Firefox.
Firmware installation fails or times out
  • On the NanoC6, hold the button on top while you connect the USB cable, then start the installation. Other boards have their own download-mode button; follow the board maker's instructions.
  • Try a shorter USB cable; long cables can cause communication errors.
  • Close any other serial monitors or programs that may be using the port.
  • Refresh the flash page and try again.
"Browser does not support Web Serial" message
  • Use a Chromium-based browser: Chrome, Edge, or Brave on desktop.
  • Safari and Firefox do not support Web Serial.
  • Web Serial is not available on iOS or Android browsers; install the firmware from a desktop computer.

Wi-Fi and connectivity

Setup Wi-Fi network does not appear

These hotspot checks apply to ESPHome and HomeKit-compatible firmware. A newly flashed Matter controller has no setup hotspot; use the Matter setup guide to add it over Bluetooth.

  • Wait at least 60 seconds after turning on the controller. The setup Wi-Fi network may take longer to appear on the first startup.
  • Make sure the controller is powered by a stable 5 V supply rated for at least 1 A.
  • ESPHome: Look for a Wi-Fi network named serin-XXXXXX (password: serinlabs).
  • Apple Home: Look for a Wi-Fi network named Serin-XXXX (password: serinlabs).
  • If the controller was previously configured, it may already be connected to your Wi-Fi network. Check your router's connected-device list, sometimes called the DHCP client list.
Controller is connected to Wi-Fi, but its IP address cannot be found
  • Check your router's connected-device list for a controller named serin or similar.
  • Use a network scanner like Fing or nmap.
  • ESPHome: The ESPHome dashboard in Home Assistant displays the controller's IP address automatically.
Controller repeatedly disconnects from Wi-Fi
  • Check the signal strength. The controller may be too far from your Wi-Fi router or access point, especially when installed inside the indoor unit.
  • Metal air-handler cabinets (SVZ, MVZ, PEAD, PVA) can weaken or block Wi-Fi signals. Mount the controller outside the sheet-metal cabinet using a longer cable; see the parts page.
  • Use a 2.4 GHz network. ESP32 controllers do not support 5 GHz Wi-Fi.
  • Apple Home: The backup setup network becomes available after the controller has been disconnected for five minutes.
Cannot open the setup page at 192.168.4.1
  • Temporarily turn off cellular data so your phone remains connected to the setup Wi-Fi network.
  • Temporarily disable automatic connections to other Wi-Fi networks.
  • For the ESPHome or HomeKit setup hotspot, or an already configured Matter controller's recovery hotspot, open http://192.168.4.1 in your browser. A newly flashed Matter controller receives Wi-Fi credentials through its smart-home app.

ESPHome and Home Assistant

Controller does not appear as "Discovered" in the ESPHome dashboard
  • Verify that the controller is connected to Wi-Fi. Check your router's connected-device list for a controller named serin.
  • Make sure the controller and Home Assistant are connected to the same local network (the same subnet or VLAN).
  • Make sure your router or firewall allows Bonjour (mDNS) traffic. Some enterprise networks and certain UniFi configurations block this traffic.
  • Try restarting the ESPHome add-on in Home Assistant (Settings → Add-ons → ESPHome → Restart).
  • If the controller is still not detected, add it manually by IP address in the ESPHome dashboard.
Installation fails or times out after selecting "Adopt"
  • Confirm that the controller is reachable on the network by pinging its IP address.
  • The first firmware build can require significant memory. Make sure the Home Assistant host has enough available resources.
  • If the wireless update fails, connect the controller over USB and reinstall the firmware from the flash page. Then select Adopt again.
Climate entity is marked "unavailable" in Home Assistant
  • Verify that the indoor unit has power. The controller communicates through the indoor unit's control board.
  • Check that the CN105 cable is firmly seated at both ends. Turn off power at the breaker before unplugging or reseating the CN105 end.
  • Check the ESPHome logs for serial communication (UART) errors. Open the controller's IP address in a browser or use the ESPHome dashboard logs.
  • Ensure TX/RX wires are not swapped; refer to the wiring reference.
Wireless firmware update fails in ESPHome
  • Confirm that the controller is reachable on the network by pinging its IP address.
  • Make sure the configuration fits within the controller's available flash storage.
  • If the update still fails, connect the controller over USB and reinstall the firmware from the flash page.

Apple Home (HomeKit)

Controller does not appear in Apple Home during pairing
  • Make sure your iPhone or iPad is connected to the same Wi-Fi network as the controller.
  • Open the web interface at http://<controller-ip> and scan the QR code.
  • If the controller was previously paired, remove it from the Home app, then select Reset HomeKit Pairing in the web interface. See reset pairing.
  • Restart the Home app and try again.
"Accessory Not Found" or pairing fails
  • Reset pairing from the web interface, then try adding the accessory again.
  • Make sure your router or firewall allows Bonjour (mDNS) traffic.
  • Restart the controller. If it is already installed, turn the power off and on at the breaker rather than unplugging the CN105 connector while the indoor unit is powered.
  • Close and reopen the Home app on your iPhone or iPad.
Web interface does not load
  • Open http://Serin-XXXX.local, using the same XXXX shown in the Serin-XXXX network name. You do not need to look up the IP address.
  • If the .local address does not open, find the controller's IP address in your router's connected-device list and open http://<controller-ip>. Some Android devices and networks block mDNS.
  • Use http://, not https://.
  • If the controller lost its Wi-Fi connection, it may have returned to setup mode. Connect to the Serin-XXXX network and open http://192.168.4.1.

Google Home, Alexa, and SmartThings (Matter)

The app cannot find the controller during Matter setup
  • Confirm you installed the Matter-compatible firmware. HomeKit-compatible and ESPHome builds use different setup routes.
  • Keep your phone near the controller: initial setup uses Bluetooth. There is no separate setup hotspot on a newly flashed Matter controller.
  • Check that you have a compatible hub for your app and that IPv6 is enabled on your home network. Follow the requirements in the Matter setup guide.
  • Start the app's add-device flow and use this controller's Matter QR code or 11-digit code. The flasher displays it after installation, and the controller prints it in the serial console at startup.
The app warns that the Matter accessory is uncertified

This is expected: the Matter-compatible firmware is a beta build and is not Matter-certified. Confirm that you want to continue in the app's uncertified-accessory prompt. Follow the instructions for Google Home, Alexa, or SmartThings.

Matter pairing finishes, but the indoor unit does not respond

Pairing confirms the smart-home connection; check that commands reach the indoor unit too. Open the controller's web interface and try a mode or temperature change. If the indoor unit still does not respond, follow the CN105 checks. Fan and vane controls are available in the web interface even when the app's standard thermostat tile does not expose them.

Indoor unit and CN105 connection

CN105 is connected, but no data appears or the climate entity is unavailable
  • Verify cable orientation; check the wiring reference for correct pin mapping.
  • Make sure the indoor unit has power at the breaker.
  • Confirm your unit has a CN105 port (not CN110). Check your model.
  • Some indoor units answer only at 9600 baud, including SEZ-KD ducted units sold outside North America. On HomeKit-compatible and Matter-compatible firmware, set Device > Advanced > CN105 Baud Rate to 9600 in the web interface. On ESPHome, set baud_rate to 9600 in the uart block of your YAML.
  • Check the controller logs for serial communication (UART) errors.
  • Try reseating the cable at both ends. Turn off power at the breaker before touching the CN105 end, and restore power only after the connector is fully seated.
  • If you assembled your own cable, an incorrectly wired or poorly soldered connection is a common cause. Test with a preassembled cable to determine whether the original cable is causing the problem.
Temperature readings are incorrect or missing
  • Some units report 0°C until the compressor runs for the first time; this is normal.
  • For more accurate room temperature, use a remote temperature sensor with the supported route for your firmware instead of the unit's built-in sensor.
  • Verify that the indoor unit is compatible and communicating. Other climate controls should respond.
Commands are sent, but the indoor unit does not respond
  • Some indoor units require at least 30 seconds after startup before accepting serial commands.
  • Check TX/RX wiring: the two data wires may be swapped. Turn off power at the breaker before opening the unit, then compare the wiring against the wiring reference.
  • If the controller's web interface loads or its logs are available, the CN105 port is supplying power to the board. This usually indicates a problem with the data wiring rather than the power supply.
  • If the board operates only when connected over USB, the cable is the likely cause. Test with an assembled cable to determine whether the original cable is causing the problem. Turn off power at the breaker before changing the cable.

Firmware and recovery

How to reset the controller to factory settings

On HomeKit-compatible and Matter-compatible firmware, select Device > Advanced > Factory Reset… in the web interface. The controller can stay in the indoor unit. For ESPHome, or a controller you cannot reach on the network, select Erase everything first in the flash tool before installing. Either way, stored Wi-Fi credentials and pairings are removed and setup starts over. See factory reset for the steps and for smaller resets that keep your pairing.

How to switch between firmware versions

Choose the destination in choose your firmware, then follow switching firmware. Use USB with the controller disconnected from CN105, erase the previous settings, and complete the new platform's setup. ESPHome, HomeKit-compatible, and Matter-compatible firmware have separate setup guides.

OTA update fails or the controller does not respond
  • Installing the firmware over USB is the standard recovery method. Follow USB recovery to disconnect the controller from CN105 safely and use the flash page. Select Erase everything first only if you intend to reset stored settings.
  • HomeKit-compatible and Matter-compatible firmware: The firmware supports automatic rollback. If validation fails after an OTA update, the controller returns to the previous firmware.
  • If the controller does not respond over USB, put it in download mode: on the NanoC6, hold the button on top while you connect the USB cable, then install the firmware.
The version did not change after an update

Reload the web interface first: Installed keeps showing the old version until the page reloads. If it still shows the old version, either the upload did not finish or the new version did not start correctly and the controller returned to the previous one. Try the update again and keep the page open until it shows "Update complete! Restarting...", or update over USB. If it happens again, contact support with the text from Copy Diagnostics.

Check for Updates fails
  • A message ending manifest missing install data, install manually below means no release is published for this board, which is usual for a DIY build. A custom build shows no Check for Updates button at all. Update either one with the flash tool or your own firmware file.
  • A Check failed message means your phone or computer could not get the release information. The controller does not need internet access, but the phone or computer you use does. Check its connection and try again, or install a firmware file.

Still need help?

Include the following information when requesting support:

  • Your indoor unit model, found on the sticker under the front panel (for example, MSZ-GL06NA).
  • The firmware and version you installed: ESPHome, HomeKit-compatible, or Matter-compatible.
  • The step at which the problem occurs: firmware installation, Wi-Fi setup, pairing, or control.
  • On HomeKit-compatible or Matter-compatible firmware, the text from About > Copy Diagnostics in the web interface. See copy diagnostics for support.
  • The troubleshooting steps you have already tried.
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