Error 801, outdoor temperature sensor fault (for weather-compensated systems).

Safety: Turn off power before working on low-voltage wiring.

• Locate outdoor sensor: Usually mounted on a north-facing exterior wall away from direct sun.
• Inspect cable: Check for cuts, crushed sections, or loose connections at both the sensor and furnace terminals.
• Replace sensor if needed: If the cable is intact but the error persists, replace with a genuine Ariston outdoor sensor of the correct resistance curve.