F13 displayed, furnace may run at reduced output or shut down

Safety first: Turn off power before working on external wiring.

• Step 1 – Locate outdoor sensor: It is usually mounted on a north‑facing exterior wall away from direct sun. Trace the cable back to the furnace.
• Step 2 – Inspect sensor and cable: Check for physical damage, UV‑cracked insulation, or water ingress at junction boxes. Repair or replace damaged cable.
• Step 3 – Resistance test: Disconnect the sensor and measure its resistance. Compare with the Protherm NTC chart. Replace if readings are incorrect.
• Step 4 – Disable weather compensation (temporary): If allowed by the control menu, temporarily disable outdoor compensation to restore basic operation until the sensor is replaced.