Furnace displays F13, heating may work intermittently, temperature readings appear incorrect

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

• Locate outdoor sensor: If your Saunier Duval system uses weather compensation, the outdoor sensor is usually mounted on a north-facing external wall away from direct sunlight.
• Inspect cable: Check the sensor cable for physical damage, UV degradation, or rodent bites. Repair or replace damaged sections.
• Measure resistance: Disconnect the sensor and measure its resistance at ambient temperature. Compare with the Saunier Duval NTC curve. An open circuit or very low resistance indicates a faulty sensor.
• Check connections: Ensure the sensor wires are firmly clamped in the control terminals and that polarity (if required) is correct.
• Replace sensor: Fit a genuine Saunier Duval outdoor sensor if defective. Reset the appliance and verify that outdoor temperature is displayed correctly (if supported).