Boiler displays E16, outdoor temperature not recognized, weather compensation not working.

Safety: Isolate power before working on wiring. Outdoor work may require a ladder; take appropriate precautions.

• Locate outdoor sensor: Find the Defro outdoor temperature sensor on an external wall, usually on the north or shaded side.
• Inspect cable: Check the cable for cuts, UV damage, or rodent damage along its route to the boiler.
• Check terminals: At the boiler, open the control panel and verify that the outdoor sensor wires are firmly clamped in the designated terminals.
• Bypass test (for technicians): A professional can temporarily substitute a known-good resistor to simulate a valid temperature and confirm sensor failure.
• Replace if needed: If the sensor is faulty, replace it with a genuine Defro outdoor temperature sensor of the correct resistance curve.