Dettson furnace shows E12 and shuts down; may occur during long heating cycles.

Safety first: Turn off power before working on sensors or wiring.

• 1. Locate return air sensor: On Dettson systems with advanced controls, a sensor may be installed in the return duct or near the blower inlet.
• 2. Inspect wiring: Check for loose or damaged wires between the sensor and control board. Repair any breaks and secure connections.
• 3. Verify sensor location: Ensure the sensor is in the main return airflow, not behind insulation or in a corner where air is stagnant.
• 4. Test and replace sensor: Measure resistance and compare to Dettson specs. Replace the sensor if readings are out of range.
• 5. Control board check: If a known-good sensor still triggers E12, the control board may have a defective input channel and should be replaced.