Vaillant smart thermostat display shows F.01 and cannot communicate with the boiler; heating does not start.

Safety first: Switch off power to the boiler and the thermostat at the main isolator before touching any wiring. Confirm power is off with a suitable tester.

Step-by-step checks:

• 1. Inspect eBUS wiring: Remove the thermostat from its wall plate or wiring center. Check the two eBUS conductors from the thermostat to the boiler (often marked eBUS, +/- or 7/8 on Vaillant boilers). Look for loose screws, damaged insulation, or crushed cable. Re‑terminate any loose wires and ensure copper is fully under the clamp.
• 2. Verify polarity and terminals: On Vaillant smart thermostats (e.g. sensoHOME, sensoCOMFORT, vSMART), ensure the two eBUS wires are on the correct eBUS terminals at both ends. While eBUS is polarity‑insensitive on many models, mis‑wiring into 230 V terminals can destroy the interface. Confirm they are not connected to 230 V L/N.
• 3. Check continuity: With power still off, disconnect eBUS at both ends and use a multimeter to check continuity of each conductor. Repair or replace the cable if open circuit is detected.
• 4. Look for shorts: Measure resistance between the two eBUS conductors; if it reads near 0 Ω, there is a short. Inspect junction boxes and cable runs for staples or crushed points and repair as needed.
• 5. Test with a short patch lead: If possible, temporarily connect the thermostat to the boiler with a short known‑good 2‑core cable near the boiler. If F.01 clears, the original cable run is faulty.
• 6. If still faulty: The thermostat’s eBUS interface or the boiler’s eBUS board may be defective. At this point, contact a Vaillant‑approved technician for further diagnosis and replacement.