Bootloader but no Teensy as Serial Device


New member
Hello all,

I apologize if this has been asked and answered, I couldn't find another post asking this specific question. I have a custom T3.6 UAV flight computer, and have been flying a few of them for about a year. I am trying to revive another board I have that I've never been able to bring to life. After solving several power problems, I now get the following symptoms:

Upon power up, connected via USB, no device appears on computer's serial bus
Pressing and holding the reset button for a few seconds causes the teensy boot loader to appear as a serial device
Uploading a blank Arduino sketch opens the teensy loader, the sketch appears to load and the teensy loader window says "reboot ok".
After uploading sketch, no teensy device appears.

I cannot ever get the teensy itself to appear as a serial device, and attempting to upload the sketch again raises the warning message that "No teensy boards were found on any USB ports of your computer." Other boards work great and are flying flight code, but for some reason this board won't let me program it. Is this a common error? Does anyone have any ideas about troubleshooting next steps? I have extra bootloader chips from the PJRC store and K66 chips, so I can try replacing one or both, but was hoping there was an easier solution that doesn't subtract from my K66 supply.

I appreciate the help,
Check the build menu for USB Type: Serial when building the blink app.

If no type including USB Serial is selected the board will not show as a serial device.

With that, a healthy Teensy should appear as a Serial device, given a good cable.

Pressing the button and getting an upload suggests the Teensy and cable are usable, assuming you uploaded a BLINK sketch or something showing it is running.

This is some old Blink before DigitalToggle.
If it uploads for about 3 seconds it will fast blink waiting for Serial connect to computer - then blink once per second and if connected prints from code should show:
#define qBlink() (digitalWriteFast(LED_BUILTIN, !digitalReadFast(LED_BUILTIN) ))

void setup() {
  digitalWriteFast(LED_BUILTIN, 0);
  Serial.begin(9600);  // USB
  while (!Serial && millis() <= 3000) if (!(millis() % 200))  qBlink();
  Serial.print("Hello World! ... ");
  digitalWriteFast(LED_BUILTIN, 1);

void loop() {
  Serial.println("Hello World");
In many of its modes, Teensy uses HID protocol rather than Serial. MIDI, Audio, and MTP (all without being Serial) are also possible.

If you are used to Arduino boards which always uses Serial protocol and you look for a serial device (Mac, Linux) or COM port (Windows), it's easy to believe Teensy may not be working. But in fact it is just using a different protocol other than Serial.

In particular, the "Flight Sim Controls" mode that works with X-Plane is HID protocol.
So I've finally had some time to try a few other things. I still can't get anything other than the bootloader to show up as an available port, even using raw HID and All of the Above options in Arduino. When anything other than serial is selected, there is an emulated serial port. Opening a serial monitor with it selected quickly changes to "TeensyMonitor:Closed" . I tried a blink sketch with the indicator LED on my board and I can't get it to illuminate. Teensy loader indicates the upload was successful but it doesn't look like its running. I can try to re-flow the K66 and see if that helps at all. if the K66 had an issue or was DOA, would the upload be successful? I appreciate the thoughts and any other ideas from you all.