A customer asked about this so I'm copy-pasting it here from a different thread for posterity:
Uwe said:
LED behavior is as follows:
During normal operation, both LEDs work together, so that there is no need to decide which one is is which, something that would be difficult on this interface due to the symmetry of the LEDs. In any kind of steady state, the LEDs blink OFF for ~100ms or so every ~1s. This indicates that the software in the interface is alive and running. If there is no blinking OFF, that's an indication that the software has crashed and that the interface needs to be rebooted.
Steady-state: LED Colors:
Yellow: Interface is plugged into car, but not enumerated via USB on a PC.
Blue: Interface is enumerated via USB on a PC, but does not see any power from the car.
Red: Indicates a fault with the connection on the car (generally a stuck K-Line).
Green: Ready to use (enumerated via USB, and plugged into a car with no obvious faults on the diagnostic socket)
During active communications, the LEDs will alternate red/green in synch with data transmissions, much like the older interfaces did.
During boot-up, just one of the LEDs will flash green for about 2 seconds (while the other LED remains off).
To further elaborate, here is some normal behavior observed on a HEX-V2.
DLC end unplugged, Plug USB end into a wall wart to provide only 5V of power, no signal:
One LED blinks GREEN for 2 seconds and then turns off.
All LEDs stay off after that.
DLC end unplugged, Plug USB end into USB port on computer
One LED blinks GREEN for 2 seconds and then turns off.
Then both LEDs blink BLUE slowly for all eternity. *See note
DLC end plugged into car, USB end plugged into USB port on computer
One LED blinks GREEN for 2 seconds and then turns off.
Then both LEDs blink GREEN slowly for all eternity.
DLC end plugged into car, USB end plugged into a wall wart to provide only 5V of power, no signal:
One LED blinks GREEN for 2 seconds and then turns off.
Then both LEDs blink YELLOW slowly for all eternity.
DLC end plugged into car, USB end unplugged:
One LED blinks GREEN for 2 seconds and then turns off.
Both LEDs blink GREEN briefly.
Then both LEDs blink YELLOW slowly for all eternity.
* Note If the HEX-V2 only has ONE blue light when plugged into the computer make sure the USB chipset drivers are not out of date.
Internal ref to ST?ID=104217