Furnace attempts to start but stops; LED shows code 12 (blower on after power up). Blower may run briefly after power is restored.

Note: Code 12 on many Day & Night furnaces is informational and can be normal after power is restored.

• Allow a full cycle: After restoring power, wait 3–5 minutes to see if the furnace initiates a normal ignition sequence (inducer starts, pressure switch closes, ignitor glows).
• Check thermostat call: Ensure the thermostat is set to HEAT and the setpoint is above room temperature. Verify the thermostat fan setting is on AUTO, not ON, if you only want heat.
• Inspect blower wiring: With power OFF, remove the blower compartment door and verify the blower motor harness and speed taps are firmly connected to the control board (HEAT, COOL, PARK terminals).
• Power reset: Turn power OFF for 60 seconds, then ON. If the furnace runs normally afterward, code 12 can be ignored.

If code 12 persists with no heat and no other codes appear, the control board logic or blower relay may be faulty and should be evaluated by a technician.