Home Assistant Yellow has 3 LEDs. The red and green LED's behavior is similar to the red and green LED on a Raspberry Pi. The yellow LED is specific to Home Assistant Yellow. It indicates Home Assistant specific information.

Power LED (red)

The power LED indicates whether the CM4 is powered. If the LED is off, check the power supply. If the LED continues to be off, remove power, and check if the CM4 is properly seated on the Yellow PCB.

Activity LED (green)

The activity LED indicates various firmware and boot loader states at startup. If CM4 fails to boot, the pattern indicates what the reason is. Refer to the LED Warning Flash Codes table of the Raspberry Pi documentation Once Home Assistant OS has been started, the green LED indicates activity. Activity is defined as disk (eMMC/NVMe) or any CPU load.

Status LED (yellow)

Home Assistant OS

Pattern Description
Off System is in EEPROM firmware or boot loader mode.
Heartbeat Home Assistant OS is running.

Home Assistant OS Installer for Yellow: LED patterns during installation

Yellow LED Description
Off System is in EEPROM firmware or boot loader mode.
Heartbeat (typically ~45s) Installer is booting.
Solid on (typically 3s) Installer successfully booted and was able to connect to the Internet.
Steady blinking (typically ~3-5 min, depending on Internet connection speed). Downloading Home Assistant OS and installing onto the eMMC or NVMe.
Off Installer successfully finished. Green LED will blink a few times before turning off. Only the red LED remains on.