Boiler displays F13, may still operate in limited mode

Safety: Work at height safely when accessing outdoor sensors.

• Locate sensor: Find the Viessmann outdoor sensor on a north-facing wall away from direct sun and heat sources.
• Visual check: Inspect for physical damage, water ingress, or loose cable glands.
• Wiring continuity: Check the cable from sensor to boiler for breaks or joints exposed to moisture.
• Sensor replacement: If F13 persists, replace the outdoor NTC with a genuine Viessmann part.
• System operation: The boiler may revert to fixed temperature control without the sensor, but weather compensation will be disabled until repaired.