Furnace will not start at all; thermostat appears to be working; LED shows normal idle code.

Safety: Turn off power before moving thermostat wires.

Steps:

• Remove the thermostat from its base and inspect the wiring. Ensure the W (heat) and R (power) wires are securely attached.
• At the furnace control board, disconnect the thermostat wires from R and W. Temporarily jumper R to W with a short piece of wire. Restore power. If the furnace starts, the problem is in the thermostat or thermostat cable.
• If the furnace still does not start with R–W jumpered, the issue is in the furnace control or safety circuits.
• Replace the thermostat if wiring is intact but it does not send a call for heat.