Thermostat calls for heat but furnace does nothing; LED on York board is steady (no fault).

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

• Check thermostat settings: Ensure mode is HEAT and temperature is set above room temperature. Replace thermostat batteries if present.
• Inspect low-voltage fuse: On many York boards, a 3–5A automotive-style fuse protects the 24V circuit. If blown, replace with the same rating. If it blows again, there is a short in the thermostat wiring or external controls.
• Test W circuit: At the furnace, temporarily jumper R to W on the control board with a short piece of wire. Restore power. If the furnace starts, the problem is in the thermostat or wiring. If it does not, the board may be faulty.
• Check wiring runs: Inspect thermostat cable for staples through the insulation or damage near the furnace and thermostat.