Furnace displays E12, runs briefly, then shuts down with no heat reaching the home.

Safety: Turn off power before accessing the return air section.

Steps:

• Locate sensor: The return air temperature sensor is usually mounted near the return air inlet or inside the blower compartment.
• Inspect and reseat: Check the sensor body and wiring for damage. Reseat the connector on both the sensor and the Atlantic control board.
• Measure resistance: Compare the sensor resistance at room temperature to the Atlantic specification chart. Replace if readings are out of range.
• Check placement: Ensure the sensor is not touching hot components or located where it can be influenced by supply air leakage.

After repair: Restore power, clear the error by cycling power if needed, and verify stable operation.