What just happened? Teensy 4.0 blinks red four times, now dead


Active member
I have been using my Teensy with USB cable without issues. Today I booted up and the Teensy blinks red four times, pause, blinks four times, etc.

I held the program button for 15 seconds and the blinking stopped, but the Teensy is still not responding or being recognized by the computer.

What happened? My setup did not change, I just turned it off and woke up today with this issue.
4 blinks means something is wrong with the flash memory chip.

Irrecoverable I'm guessing? Not sure what I did. Would electrostatic damage be a reasonable hypothesis, or just a bad unit? Just want to make sure when I plug in a new one I'm not going to fry that one too.
Another Teensy dead. This time I pressed the Program button and it was one long red, on and off. Reconnected it and now it's no LEDs, totally dark, dead and Windows says "USB device has malfunctioned."

I'm grounding myself every time I touch the thing. I've triple checked my circuit. I don't know how else to troubleshoot this.
Last edited:
To start troubleshooting, first get a DC voltmeter. Measure the voltage between VIN and GND. It should be 5 volts, from USB. Next measure the voltage from 3.3V to GND. If Teensy powered up, it should be 3.3V.

If your Teensy is powered up, try holding the pushbutton for 15 seconds. At 13 seconds it should give you a quick blink on the red LED (near USB) to indicate the beginning of the 4 second window where releasing the button initiates a full wipe and restore of the flash memory. Let the button go when you see that. The red LED is supposed to turn on bright for quite some time as it slowly wipes the flash. On Teensy 4.0 is usually takes about 45 seconds. Let it finish. If the process works, when the red LED turns off Teensy will reboot and it will be running the LED blink program again, which blinks the orange LED.
As for what went wrong, almost impossible to guess from only this info. Just a defective board doesn't make sense if 2 have failed. Likewise for ESD, unless you have a really horrible environment (you would know as your body would feel shocks regularly when touching things) two failed seems pretty unlikely.

All sorts of "software" things can go wrong. If it's not actual damage to the hardware, that 15 second button press should always recover.

I know you said you triple checked your circuitry. But when multiple boards fail, it's almost always something about the circuitry connected to them. If this circuitry isn't secret, maybe showing us the details could help get more eyes looking for the problem.
Thanks Paul, I was able to recover one of them using your information. Three of them are still dead (not showing 3.3v). I can attribute two of them to accidental swipes of 12v wires, but the one that had the flash memory error remains a mystery as to why that zonked out. I have quarantined the board to a corner where it's not to be touched—if another Teensy dies on it with no interaction I will report back with details on the circuit.