2.2 - Status LED [U3 Datasheet]

There is a green status LED on the LabJack U3. This LED blinks on reset, and then remains steadily lit. Other LED behavior is generally related to flash upgrade modes (Section 1.2).

Normal  power-up Status LED behavior: When the USB cable is connected to the U3, the Status LED blinks 5-6 times over 2 seconds and then remains solid on.

Normal power-up Status LED behavior but device keeps rebooting, even with no connections besides USB:  You might notice this because you see the normal power-up LED behavior happening repeatedly and perhaps also hear the host making noises as the device disconnects and reconnects.  Likely indicates that the Watchdog is resetting the device.  To reset to factory default values (i.e. disable watchdog), power up the U3 with a jumper securely installed from FIO6 to SPC (FIO2 to SCL on U3 1.20/1.21), then remove the jumper and power cycle the device again.

Status LED blinking continuously at about 4 Hz, even with no software running: This indicates that the U3 is in flash mode. See Section 1.2 and reprogram the device.

Other or no Status LED behavior, even with no connections besides USB:  Try using the different jumpers and reprogramming as described in Section 1.2.  If you get no LED activity with any jumpers, but know the USB port is good and providing 5 volts, the device is likely damaged beyond field repair possibility (returns, exchanges, and repairs).  Also see item F in the USB Communication Failure App Note.