Error 802, room thermostat or bus communication fault.

Safety: Isolate power before changing thermostat wiring.

• Verify wiring: Check that the room thermostat or Ariston bus controller is wired according to the furnace manual (polarity-sensitive on some models).
• Check cable continuity: Inspect the cable for damage along its route. Repair or replace if necessary.
• Test with simple jumper: For on/off thermostats, temporarily bridge the thermostat terminals at the furnace to see if it responds. Remove the bridge after testing.