Ensure the battery contacts are clean and there is no dirt or oxidisation on the contacts. (The ZEN Thermostat uses 4 x AA alkaline batteries installed in the back of the display. Remove your ZEN Thermostat from the wall to access the batteries.)
If there is any oxidation, use an electrical contact cleaner and gently clean the battery terminals. Do not spray the contact cleaner directly on the thermostat rather spray it on a cloth and then wipe the terminals with the cloth.
Please note: Low Battery Icon can also mean that there is a battery inserted into the Zen in a reverse manner. Please ensure proper battery orientation