Furnace control board LED flashes 1 short and 1 long (code 11); furnace will not start a heat cycle.

Safety first: Turn off power to the furnace at the service switch and breaker before touching any wiring or components.

• Step 1 – Verify power supply: Check the furnace service switch (usually on or near the furnace) and ensure it is in the ON position. Check the dedicated furnace breaker in the electrical panel and reset if tripped.
• Step 2 – Inspect low-voltage connections: Remove the blower door to access the Tempstar control board. Confirm the door switch is being fully depressed when the panel is installed. Inspect the 24V transformer leads and thermostat wiring at the R and C terminals for loose or broken wires. Tighten any loose screw terminals.
• Step 3 – Cycle power: Turn power off for 30 seconds, then restore power to clear the code. Observe the LED. If the furnace starts normally, the code was informational.
• Step 4 – Check for recurring power loss: If the code reappears frequently, inspect the furnace service switch, wiring harness to the control board, and the 24V transformer for intermittent connections or overheating.
• Step 5 – Replace control board if needed: If proper power is present (120V to line terminals and 24V between R and C) but the board will not initiate a call for heat, the Tempstar integrated furnace control board may be defective and should be replaced by a qualified technician.