E12 error and thermostat cannot detect external temperature sensor (for outdoor compensation).

Safety first: Turn off power to the thermostat and any associated control circuits before working on sensor wiring.

• Step 1 – Verify configuration: In the installer menu, confirm that Outdoor sensor is enabled only if one is actually installed. Disable it if not used to clear E12.
• Step 2 – Inspect sensor location: Locate the outdoor sensor on the exterior wall. Ensure it is mounted out of direct sun and rain, and that the housing is intact.
• Step 3 – Check wiring: Inspect the cable from the sensor back to the thermostat or control box. Tighten any loose terminals and repair visible damage.
• Step 4 – Measure resistance: With power off, disconnect the sensor and measure its resistance. Compare to Atlantic’s NTC curve. Infinite or near‑zero resistance indicates a failed sensor.
• Step 5 – Replace sensor: Install a new Atlantic‑compatible outdoor NTC sensor, observing polarity if specified. Seal any external junction boxes to prevent water ingress.
• Step 6 – Clear error: Restore power and verify that the thermostat now shows a realistic outdoor temperature and E12 is cleared.