DIAGNOSIS Unstable or low voltage from the HVAC system, often due to a poor connection at the C-wire or a system sharing the transformer with another device.
FIX Pull the display off. Check all wire connections on the Nest baseplate. Ensure each wire is straight, fully inserted, and the terminal screw is tight. A loose C-wire is the most common culprit for flickering.
IF THAT DIDN'T WORK Your HVAC system's transformer may be undersized or failing. Disconnect other low-voltage devices like doorbells or zone controllers from the same transformer. Test if the flickering stops. If it does, the transformer needs upgrading.
HEADS UP Persistent flickering can corrupt the Nest's software. If flickering continues after checking connections, perform a Settings > Reset > Restart to prevent a software crash.