RuuviTag (standard and RuuviTag Pro) has two leds, green and red. During power-up, red light is on for the duration of self-tests and green led lights up for a second after a successful self-test and boot.
During normal operation, green led blinks shortly when new measurement is taken – 2.5 seconds by default.
Configuration mode (also called bootloader mode)
Green led lights up when a button is pressed. If you hold button down for 10 seconds, red led lights on and RuuviTag is in bootloader mode, ready to accept new firmware.
After button press or NFC read RuuviTag stays in configuration mode for 60 seconds and red led is blinking once per every measurement. During configuration mode it is possible to command RuuviTag into bootloader via Bluetooth. Usually this is not needed, but if you have a special case where you cannot reach the buttons of RuuviTag and need to update the firmware contact Ruuvi support for more details.
LED indicators on various firmware versions
Find details regarding LED indicators on different firmware versions here.