You can say 'Hey Google' whether your screen is on or off.
You can say 'Hey Google' while you use Google Maps and Android Auto. This setting could prevent some other voice services that use hot words or wake-up commands from working. If the screen is on or the device is charging, you can say 'Hey Google' from any screen. Learn how to change your Google app settings. Not all settings are available for all languages. Settings can vary by device and Google app version.