This can have different resons and needs some investigation:

  1. Connect USB to your FC
  2. Open up BlHeli Tool (newer than Version 16.4)
  3. Select Silabs / Cleanflight interface
  4. Power up the ESC
  5. Select the Flightcontrol as COM port and make sure Beta/ Cleanflight tool is no more connected to your FC!
  6. Hit "check"

Now your ESC should get listed, if not, then there is something wrong.

ESC get listed but not all of them:

  • Is the ESC working at all? Does it work in motors tab? Will the motor spinin motors test tab?
    • If that is the case then check if the ESC is working when connecting to another Flightcontrol PWM output. Is it working there? If so your FC might be damaged (usually by accidently shorting PWM to battery voltage)

ESC is not working or responding in motor test tab

  • check the communication protocol, set it to oneshot and test again. 

ESC still not working at all

it could have the following reasons:

  • No valid PWM signal from FC on ESC PWM input
  • Bootloader on the ESC has been deleted and it needs to get reflashed using a silabs C2 interface (could happen with some older BlHeli versions around 16.2, other than samples to testpilots, TBS never shipped ESC with that affected version
  • ESC got no input voltage or lower than 7V
  • ESC got no internal 5V most propably due to a ripped off component in a crash (can be checked at the 3.3v regulator on board for the technicians)
  • Something shorted or damaged during remove of the capacitor for Dshot modd.