Vaillant smart thermostat shows F.19 and reports low battery or power supply issues.

Safety: For mains‑powered units, isolate power before removing from wall plate.

• 1. Replace batteries: For battery‑powered room units, replace all batteries with new, high‑quality alkaline cells of the specified type. Do not mix old and new batteries.
• 2. Check battery contacts: Ensure the contacts are clean and not corroded. Gently clean with a dry cloth if needed.
• 3. Verify low‑voltage supply: For wired thermostats, measure the supply voltage at the terminals and confirm it matches Vaillant specifications. Investigate the boiler or external PSU if low.
• 4. Reseat thermostat: Remove and re‑attach the thermostat to its wall plate to ensure a firm connection.
• 5. Replace PSU or thermostat: If supply is unstable or F.19 persists with good batteries, the PSU or thermostat power circuitry may be faulty and require replacement.