Outdoor temperature sensor error on systems with Viessmann weather-compensated control, heating curve not working

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

• Check sensor location: The Viessmann outdoor sensor should be on a north or northwest wall, away from direct sun and heat sources. If it is in direct sun, readings will be inaccurate.
• Inspect wiring: At the boiler control, locate the terminals for the outdoor sensor (e.g., terminals 1 and 2 on some models). Ensure the two-core cable is firmly connected and not shorted.
• Check sensor body: Inspect the sensor housing outside for cracks or water ingress. Replace if damaged.
• Bypass test (technician): A professional can temporarily connect a known-good resistor to simulate the sensor and verify control response.
• Reset control: After correcting wiring or replacing the sensor, reset the boiler and verify that the outdoor temperature reading is plausible.