Thermostat shows A1 and cannot detect outdoor temperature; weather‑compensated operation is disabled.

Safety first: Switch off power to the boiler at the main switch or breaker before inspecting wiring.

• Confirm outdoor sensor is installed: On many Viessmann systems, the outdoor sensor is wired directly to the boiler, not the thermostat. Locate the outdoor sensor on an exterior wall and verify it is present and intact.
• Check sensor wiring at boiler: Open the boiler front panel (if user‑accessible) and locate the outdoor sensor terminals (often labeled with sensor symbol or OT). Ensure both wires are firmly connected and not corroded.
• Inspect cable run: Follow the cable from the boiler to the outdoor sensor. Look for damage from UV, rodents, or mechanical impact. Repair or replace damaged cable sections.
• Compare displayed temperature: Restore power and compare the thermostat or boiler’s outdoor temperature reading to actual outdoor temperature. If it shows unrealistic values or dashes, the sensor may be defective.
• Replace sensor if needed: If wiring is sound but readings are wrong or missing, replace the Viessmann outdoor temperature sensor with the correct model specified for your boiler series.