vSMART thermostat shows a message indicating no connection to the boiler; heating and/or hot water do not respond to setpoint changes.

Safety warning: Isolate mains power to the boiler and any external controls before touching wiring. Only remove boiler covers if you are trained and allowed to do so in your jurisdiction.

Steps to check communication:

• Power cycle: Turn off the boiler at the fused spur or main switch for 30 seconds, then turn it back on. Also power cycle the vSMART receiver if it has a separate supply.
• Check eBUS cable: Locate the low‑voltage eBUS cable between the vSMART boiler interface/receiver and the boiler eBUS terminals. Ensure both ends are firmly connected and not damaged or pinched.
• Verify correct terminals: Confirm the cable is on the boiler's designated eBUS terminals (often marked eBUS, +/−, or 7/8 depending on model) and on the correct terminals on the vSMART interface.
• Inspect for corrosion or burning: With power off, visually inspect the terminals and board area for signs of corrosion, overheating, or burning. Do not touch damaged boards; call a technician.
• Reset pairing: If the vSMART uses a wireless receiver, re‑pair the thermostat with the receiver following the Vaillant instructions, then check if the boiler responds.

If communication cannot be restored or the boiler shows its own eBUS error codes, contact a qualified Vaillant engineer to test the boiler control board and vSMART interface module.