Thermostat calls for heat but furnace does nothing; control board LED is on solid, but no inducer or blower starts.

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

• Verify thermostat settings: Ensure the thermostat is set to HEAT and the setpoint is above room temperature. Replace batteries if applicable.
• Check 24V fuse: Inspect the low-voltage fuse on the control board. Replace if blown, but also look for shorts in thermostat wiring.
• Test heat call at furnace: With power off, remove the thermostat wires from R and W at the furnace. Restore power and temporarily jumper R to W at the control board. If the furnace starts, the issue is in the thermostat or thermostat cable.
• Inspect thermostat cable: Look for damage along the cable path, especially where it passes through walls or near the furnace cabinet.
• Replace thermostat if needed: If wiring is intact but the thermostat does not send a heat call, install a compatible replacement.