Safety: Isolate boiler power before working on wiring. The outdoor sensor is low voltage but may be routed alongside mains cables.

Steps:

Locate the outdoor sensor on an external wall, ideally north‑facing. Check that it is securely mounted and not directly in sun or near flue terminals.
Inspect the sensor housing for cracks or water ingress. Replace if the enclosure is compromised.
With power off, disconnect the sensor and measure its resistance. Compare to the Glow‑worm NTC chart for the current outdoor temperature.
Check the thermostat or system controller configuration to ensure outdoor compensation is enabled only if a sensor is installed and that the correct sensor type is selected.

Remedy:

Replace the outdoor sensor if resistance is out of specification or if the housing is damaged.
Repair or replace any damaged cable and ensure joints are weatherproof.
After repair, restore power and confirm that the thermostat displays a realistic outdoor temperature and that F.13 clears.

Thermostat displays F.13 and outdoor temperature is not shown

Possible Causes