Thermostat display shows error 101 and heating does not start even when there is a demand for heat.

Safety first: Turn off power to the boiler and thermostat at the main isolator before touching any wiring. Confirm with a non-contact voltage tester.

• Check bus wiring polarity: Open the boiler front panel (if user-accessible) and the thermostat backplate. Verify the two communication wires (often marked OT, BUS, or +/-) are connected to the correct terminals on both boiler and thermostat. Reverse them only if the manual specifies polarity.
• Inspect for loose or broken wires: Gently tug each conductor at the boiler and thermostat terminals. Tighten any loose screws and re-terminate any damaged copper ends.
• Check cable continuity: If you have a multimeter and are competent to use it, test continuity of each bus wire between boiler and thermostat. Replace the cable if open circuit is detected.
• Power cycle: Restore power to the boiler and thermostat, then perform a full power cycle (turn off for 30 seconds, then on). Wait 2–3 minutes to see if error 101 clears.
• If still faulty: The boiler’s communication board or the thermostat’s communication module may be defective. Do not open sealed electronic modules; contact an Ariston-authorized technician.