Smart thermostat displays F.04 and cannot detect any wireless receiver or boiler interface; heating does not respond.

Safety: Isolate mains power before opening the boiler casing or receiver housing.

• 1. Check receiver power: Verify that the Vaillant wireless receiver (often mounted near the boiler) has power. Its status LED should be on or flashing according to the manual. If dark, check its fused spur and internal fuse.
• 2. Confirm wiring to boiler: Ensure the receiver’s control wires are correctly connected to the boiler’s eBUS or room thermostat terminals as specified by Vaillant. Tighten any loose connections.
• 3. Re‑pair thermostat and receiver: On the thermostat, go to wireless settings and start pairing mode. Press and hold the pairing button on the receiver until the LED indicates pairing. Wait for confirmation on the thermostat.
• 4. Reduce interference: Move the thermostat closer to the receiver temporarily and avoid placing it near large metal objects, Wi‑Fi routers, or microwaves. Test if F.04 clears when closer.
• 5. Factory reset receiver: If pairing fails, perform a factory reset on the receiver following the Vaillant instructions, then repeat the pairing process.
• 6. Replace receiver if needed: If the thermostat cannot detect the receiver even at close range and after reset, the receiver module may be faulty and should be replaced by a professional.