Thermostat shows E11 and cannot detect external temperature or weather compensation input.

Safety first: Turn off power to the wiring centre or boiler controller before accessing terminals. Outdoor sensor circuits are usually low voltage but may share enclosures with mains wiring.

Steps to diagnose and fix:

• 1. Inspect outdoor sensor: Locate the Danfoss outdoor sensor on the exterior wall. Check for physical damage, water ingress, or paint covering the sensor housing.
• 2. Check wiring at both ends: Open the thermostat or wiring centre and verify that the sensor wires are firmly connected to the correct terminals. Tighten any loose screws.
• 3. Measure resistance: Disconnect the sensor wires and measure resistance across them. Compare to Danfoss NTC outdoor sensor tables at the current outdoor temperature. Infinite or near-zero readings indicate a fault.
• 4. Inspect cable route: Look for junction boxes or splices along the cable run. Open and check for corrosion or loose connections. Replace any water-damaged junction boxes.
• 5. Replace sensor if faulty: Install a new Danfoss-compatible outdoor sensor in a shaded, ventilated location on the north or northwest wall. Connect using UV-resistant cable and weatherproof junctions.
• 6. Reset thermostat: After repair, clear the E11 error and confirm that outdoor temperature is displayed correctly.

When to call a professional: If the sensor cable is routed through inaccessible areas or at height, a professional installer should perform the replacement.