Furnace will not start at all, but power and LED are present

Safety first: Turn off power before working on low-voltage wiring to avoid shorting the transformer.

Steps to check:

• Thermostat settings: Confirm thermostat is set to HEAT and above room temperature.
• Check wiring at furnace: Remove blower door and verify R and W wires are firmly attached to the control board terminals.
• Test thermostat circuit (for basic users): With power off, temporarily jumper R to W at the control board. Restore power. If the furnace starts, the thermostat or thermostat wiring is likely faulty.
• Replace thermostat: If wiring is intact, try a known-good thermostat compatible with your Gibson furnace.