Control board LED flashes code 12; blower runs briefly after power-up, but no heat call is processed.

Note: On many Tempstar furnaces, code 12 can be informational, indicating the blower ran after power-up. It should clear on the next call for heat.

• Step 1 – Call for heat: Set the thermostat to HEAT and raise the setpoint 5°F above room temperature. Observe if the furnace begins a normal ignition sequence (inducer starts, pressure switch closes, ignitor glows, burners light).
• Step 2 – Observe LED after a heat cycle: Once a full heat cycle completes, check if the code 12 clears. If it clears, no repair is needed.
• Step 3 – Check blower operation: If the blower runs continuously without a call for heat or cool, inspect the G terminal wiring from the thermostat and ensure it is not shorted to R. Disconnect the thermostat wires at the furnace and see if the blower stops.
• Step 4 – Inspect control board relays: With power off, inspect the Tempstar control board for burnt or stuck blower relay contacts. If the blower relay is welded closed, the board must be replaced.
• Step 5 – Verify blower wiring: Confirm the correct speed tap wires from the blower motor are connected to HEAT, COOL, and PARK terminals as per the Tempstar wiring diagram.