Thermostat calls for heat, but the furnace does nothing; the control board LED is on and steady, with no error code.

Safety: Turn off power before moving thermostat wires to avoid short circuits.

Steps:

• Verify thermostat mode: Confirm it is set to HEAT and the setpoint is above the room temperature.
• Check thermostat batteries: Replace batteries if the display is dim or blank.
• Test at furnace: Remove the blower door, locate the R and W terminals on the control board, and with the power off, temporarily connect a jumper between R and W. Restore power. If the furnace starts, the thermostat or wiring is faulty.
• Inspect wiring: Check for loose or broken thermostat wires at both the furnace and thermostat.

When to call a technician: If jumping R to W does not start the furnace, the control board or safety circuits may be at fault and require professional diagnosis.