Note: When you start or stop streaming a title in 4K Ultra HD, your TV screen possibly flickers or turns black for up to five seconds.
Restart your Fire TV device.
Use the connection equipment that came with your device.
Turn on any A/V receivers or sound bars connected to your Fire TV device and switch them to the correct input. Or connect your Fire TV device directly to the television.
Disconnect the Fire TV device from the HDMI port then plug it back in.
Try replacing your HDMI cable or HDMI hub (not included) with a new one.
Use a High-Speed HDMI cable (not included) for streaming titles in 4K Ultra HD.
Match your remote's source or input button to the HDMI port that your Fire TV is plugged into.
Turn off HDMI-CEC Device Control in Settings under Display & Sounds, then turn HDMI-CEC Device Control back on.
Cycle the available resolutions on your Fire TV device by pressing and holding the Up and Rewind buttons together for five seconds. Allow your device to cycle through all available resolutions.