Thermostat shows E4 and battery icon flashes (for battery-powered models)

Safety: Only low-voltage battery work is required; do not open the boiler.

• Replace batteries: Open the battery compartment and replace all batteries with new, high-quality alkaline cells of the same type. Do not mix old and new batteries.
• Clean contacts: If there is white or green corrosion on the contacts, gently clean with a dry cloth or a pencil eraser. Do not use water.
• Check orientation: Ensure all batteries are inserted with correct polarity (+/−) as indicated.

After replacement, E4 should clear automatically. If not, the battery sensing circuit may be faulty and the thermostat may need replacement.