Furnace will not start at all, but LED shows normal power; thermostat appears to be working.

Safety: Turn off power before moving low-voltage wires.

Steps to check:

• Bypass thermostat: At the furnace control board, temporarily jumper R to W. If the furnace starts, the thermostat or its wiring is at fault.
• Inspect thermostat wiring: Check for loose or broken wires at the thermostat and furnace terminals.
• Check thermostat settings: Ensure the thermostat is configured for gas furnace (not heat pump or electric) and that any installer setup is correct.
• Replace thermostat: If wiring is intact and bypass works, replace the thermostat with an Armstrong-compatible model.