Furnace displays E12 and will not fire, indicating an outdoor temperature sensor fault (for weather-compensated Danfoss systems)

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

Steps:

• Inspect outdoor sensor: Locate the Danfoss outdoor sensor on the north-facing exterior wall. Check that the housing is intact and not filled with water or insects.
• Check cable: Follow the cable back toward the furnace, looking for mechanical damage. Repair or replace damaged sections.
• Verify connections: At the control board, ensure the outdoor sensor wires are on the correct terminals and firmly tightened.
• Replace sensor if needed: If the sensor is physically damaged or tests open/short with a meter, install a new Danfoss outdoor sensor of the correct resistance curve.
• Reset controller: Restore power, clear E12, and verify that the displayed outdoor temperature is reasonable.