Solutions for receiver and high-frequency head module issues

I. General Troubleshooting

Q1: My receiver and transmitter are bound, but the flight controller board is not responding to inputs, and I can only see 10 telemetry sensors
A1: Check if the receiver's RX and TX pins are correctly wired. Please check if you have the correct SerialRX configuration. You may need to revisit the FC Preparation page for flight controller setup.
Additionally, please ensure that the UART you are connecting the receiver to is not inverted and is in full duplex mode. You can also try a different UART.
Q2: My receiver and transmitter are bound, the transmitter shows more than 10 telemetry sensors, but there is no joystick input
A2: This is caused by a model mismatch between the receiver and the module. The LED on the receiver will briefly flash 3 times, then pause for 1 second, indicating a mismatch. To fix this issue, you may need to enable Model Matching.
Another item to check is whether the RCVR_INVERT_TX option is enabled for the receiver firmware. This option is for flight controllers that have no other available UARTs (such as AIO flight controllers) and can only use the SBUS pin.
For R9mm/mini receivers, ensure that USE_R9MM_R9MINI_SBUS is not enabled in the firmware options. Refer to Output Inverting to understand what this option does.
Q3: My receiver and transmitter are bound, I'm getting joystick input, but I can only get 10 telemetry sensors
A3: Check if you have enabled telemetry in the flight controller configuration. Verify that the receiver's RX pad is correctly connected to the TX pad on the flight controller board, in the same UART as the receiver's TX line.
Additionally, ensure you have deleted and rediscovered sensors. It's good to increase the TLM Ratio so that sensor acquisition will be faster.
This could also mean that you must update your FC firmware to the latest version that supports at least the CRSFv2 protocol.
Q4: I think my receiver and transmitter are bound because the LED indicator on the receiver is constantly on. But I have no joystick input or RSSI on my transmitter.
A4:You are not bound. The receiver may be in bootloader mode.

1. Ensure pin connections are disconnected: If you flashed the receiver through passthrough, make sure you have disconnected the boot pads.
2. Check if the LED is still constantly on: Turn off the transmitter. If the LED is still on constantly, but the boot pads are not bridged, then the UART connected to the receiver may have unintentionally put it in bootloader mode. This is usually due to a pull-down resistor on the receiver's RX pin, which helps handle inverted SBUS lines.
3. Verify UART issue: To verify the UART issue, you can disconnect the RX and TX lines from the flight controller. If the receiver starts blinking slowly, then after a set interval (default 20s - 30s) it will start blinking rapidly, indicating it is in WiFi mode.
   Solution: 1. Connect the receiver to a different available UART. 2. If there are no other UARTs available, you can solve the problem by connecting a 300Ω to 1kΩ resistor between the 3.3V pin and the flight controller's RX pin (where the receiver TX is connected).
   If the problem persists: Even after disconnecting TX and RX lines, if the LED is still constantly on, the receiver may be soft-bricked due to a failed WiFi flash. In this case, you need to follow the unbricking procedure to recover the receiver.
   Q5: My OSD shows LQ 0:0 and RSSI dBm at -130 dBm, but I have joystick input and more than 10 telemetry sensors
   A5: Check if you have set the RSSI channel to Aux11 or Aux12. If so, set it to disabled.
   If that doesn't fix it, check if you have RSSI ADC enabled. If so, set it to disabled.
   Q6: My OSD sometimes shows LQ 3:1 and sometimes shows LQ 3:9
   A6: Move the LQ OSD data as it might be blocked by another data field or it's too far from the right side of the preview.
   Q7: I'm getting Telemetry Lost/Recovered
   A7: There are many causes for Telemetry Lost/Recovered issues. If you are using a newer transmitter/handheld device, this should not happen unless you are flying very far and using a receiver that doesn't amplify the telemetry signal (such as an EP receiver).
   Here are some things to check:
4. You are using an X9D(+) or QX7 with a not-so-ideal inverter chip. Please see this page for how to resolve this issue.
5. You are using an early version of the Happymodel Slim Pro. Please check the manufacturer's post on Facebook for the fix.
6. You are using the 2018 version of the ACCST R9M and your transmitter's baud rate is 400k or higher. In this case, you need to lower your transmitter's baud rate to 115200 (such as QX7), or perform a resistor modification on the module as described here.
7. Your module is loose inside the case, which is most common in the first batch of Happymodel ES24TX (white 3D printed case; although the black version is slightly better with 4 screw points instead of 2).
8. You need to print a new case (you can search for "expresslrs" on Thingiverse), or find a way to secure the module and the case.
9. Check the S.Port pins and connections on the module and in the module bay to ensure they have a stable physical connection. On DIY modules, especially full-size modules, the round holes/vias of the flat pins of the Molex connector may not make good contact due to cold solder joints or insufficient solder.
   Q8: Before getting my receiver and transmitter bound, I inserted my flight controller for the second/third time. When I power it on, the LED on the receiver dims
   A8: Please update to the latest firmware as soon as possible. A batch of HM receivers entered the market with insufficient voltage regulators (marked as SDG) that couldn't handle the higher current consumption during startup.
   Q9: My Blackbox logs show steps on the RC path.
   Q9: Make sure you have disabled ADC Filter in the Radio Hardware settings.

II. Troubleshooting Lua Scripts

Q1: Refreshing ExpressLRS Lua script on the transmitter is stuck at Loading...
A1: Refer to the transmitter setup guide to ensure your radio is ready for ExpressLRS. Also, make sure your module has been flashed with v3.0 firmware. V3 Lua is for modules flashed with V3.0, and V2 Lua is for modules flashed with v2.x (including some fresh out of the box).
For newly purchased ExpressLRS modules, USB flashing is the recommended update method.
Q2: Betaflight Lua is stuck at Initializing
A2: Through the ExpressLRS Lua script, increase the TLM Ratio to values like 1:4 or even 1:2, then retry the Betaflight Lua script. Additionally, before starting the script, you must ensure you have more than 10 telemetry sensors.
Q3: Cannot change switch mode on the Express LRS page on the transmitter
A3: Changing switch mode requires that the module and receiver are not connected to each other (i.e., no C in the upper right corner of the Lua script). First, turn off the receiver's power, change the Switch Mode from the Lua script, then turn on the receiver's power. Then apply the Switch Mode.
Q4: VTX Admin is not working properly, but I can change VTX channels through the OSD menu
A4: Since VTX Admin depends on MSP, which in turn depends on telemetry, please ensure you have telemetry enabled on the FC flight controller configuration, and you have more than 10 telemetry sensors on your transmitter (Model Settings -> Telemetry page); Delete and Discover New sensors to refresh the sensor list.
Q5: There is no C in the upper right corner of the ExpressLRS Lua script on the transmitter (i.e., the transmitter is not connected to the receiver), but I'm not getting telemetry on my transmitter!
A5: Make sure Telem Ratio in the ExpressLRS Lua script is not set to Off. Set it to Std or any other value instead of Off.
Q6: On v1.x versions, I could select 2W in Lua, but after updating to a new version, I can't select it anymore. What's the reason?
A6: This means your module itself cannot achieve such high or low power levels. The power levels you can select in the new Lua script are based on the power levels supported by your module.
This also applies to other options such as Enable Backpack WiFi, BLE and Joystick. If your module doesn't support these features, they won't appear in the Lua script.

III. Issues with Flashing and Updating Process

Q1: Invalid Serial RX Configuration Detected
A1: This issue is typically caused by incorrect Serial RX protocol settings (should be set to CRSF), or incorrect serialrx_inverted = on or serialrx_halfduplex = on settings. The Passthrough Init section in the log will tell you which setting needs to be corrected.

Q2: RX Serial Number Not Found
A2: Ensure that the Serial RX in the Ports tab of the FC firmware configurator is set to the correct UART.

Q3: No CLI Available
A3: This message is typically seen in the ExpressLRS Configurator Log when using the Passthrough method, and may be caused by a COM port being busy. Please ensure you close any other configurator applications and reconnect the flight controller's USB connection.
You can also try using a different USB port or USB cable. This could also mean the flight controller is not being detected by the script. This might be because the flight controller is in DFU mode, which is not required for Passthrough flashing.
Some applications might be running in the background causing this issue. For example, Cura Slicer. If you have such applications, close them and try a new Build and Flash operation. Before trying again, reconnect the flight controller's USB connection.

Q4: Update Retried Multiple Times but Failed
A4: This could be caused by the following reasons:

1. Incorrect bootloader or improperly flashed bootloader: This typically occurs with Frsky R9 receivers.
2. Incorrect wiring: Ensure the receiver's RX pin is connected to the flight controller's TX pin, and the receiver's TX pin is connected to the flight controller's RX pin. Also, ensure the receiver receives sufficient voltage (minimum 4.5V) from the flight controller or a power source.
3. Receiver turned off: Check if the LED on the receiver is lit, indicating it is powered on and operational.
4. Hardware inversion on UART: Ensure the UART you are connecting the receiver to is not an SBUS UART that typically has hardware inversion (this is common in F4 flight controllers). Some flight controllers require you to bridge a pair of pins to enable or disable hardware inversion on the RX pin. You can try connecting to another UART.
5. Receiver's LED is constantly on while the transmitter is off: This might mean the flight controller's RX pin is pulled low, causing ESP-based receivers (such as EP, ES900RX, etc.) to unexpectedly enter Bootloader mode, which will interfere with normal Passthrough operations. You can try using another UART.

Q5: Error "Failed to connect to ESP8266: Timed out waiting for packet header." during update
A5: This could be caused by the following reasons:

1. Incorrect receiver wiring: Please check if RX and TX wires are swapped, and ensure the RX pin is connected to the flight controller's RX pin, and the TX pin is connected to the flight controller's TX pin.
2. The receiver's "boot" pin is not bridged (or the button is not held during power-on) for passthrough flashing. This is required if the receiver is brand new and has a firmware version below 1.0.0-RC6.
3. The receiver's UART is putting it in Bootloader mode and interfering with passthrough flashing. Move the receiver to another UART. You can also try updating using an FTDI adapter (or USB to UART bridge) via UART.

Q6: Updated via WiFi, but now the receiver doesn't work and the LED indicator is constantly on
A6: This is a sign that the WiFi flashing did not complete properly due to premature power disconnection. To resolve this, you need to bridge the boot pin and re-flash using the Passthrough method (in Betaflight or iNav, the telemetry option is set to OFF, this is very important!), or flash using the FTDI adapter via UART method.
This video shows the steps, although it uses the VSCode development environment, the ExpressLRS Configurator will also work fine.

(This video is from YouTube channel Jye Smith)
To ensure proper WiFi flashing**, wait until the LED on the receiver blinks slowly** (indicating it is looking for TX binding) before disconnecting the power
———— This article is referenced from ExpressLRS Getting Started: https://www.expresslrs.org/quick-start/troubleshooting/#i-updated-via-wifi-but-now-the-receiver-wont-work-and-has-solid-led**