No LED lights on Armstrong Air furnace control board; furnace completely dead and unresponsive.

Safety: Turn off the breaker before checking internal wiring. Use caution when working around 120 VAC.

Steps to check:

• Check breaker and switch: Verify the furnace circuit breaker is ON and the furnace service switch is in the ON position.
• Door switch: Ensure the blower compartment door is properly installed and depressing the door safety switch. The furnace will not power up if the door is off or misaligned.
• Inspect board fuse: Locate the low-voltage automotive-style fuse (typically 3–5A) on the control board. If blown, replace with the same rating. If it blows again immediately, there is a short in the 24V circuit (often thermostat wiring or outdoor unit contactor coil).
• Check transformer output: With power on and using a multimeter, measure 24 VAC between R and C terminals on the control board. If there is no 24 VAC, the transformer may be failed or not receiving 120 VAC.
• Replace failed components: If the transformer has proper 120 VAC input but no 24 VAC output, replace the transformer. If power and transformer are good but the board remains dead, replace the control board.