Safety first: Turn off furnace power before working on low-voltage wiring.

Steps:

Remove the blower door to access the CTC integrated furnace control board.
Verify that the 3–5 A low-voltage fuse on the board is intact. Replace with the same type and rating if blown.
Inspect thermostat wires connected to R, W, Y, G, and C terminals for loose strands or shorts against the furnace cabinet.
At the thermostat, remove it from the wall plate and check that R and W are properly connected for heat.
To test the furnace, temporarily jumper R to W at the control board. Restore power. If the furnace starts, the issue is with the thermostat or thermostat wiring.
If jumpering R to W does not start the furnace and 24 V is present between R and C, the control board's heat call circuit may be defective and the CTC control board will need replacement.

Warning: Only qualified persons should replace control boards; incorrect wiring can damage components or create unsafe conditions.

Furnace does not respond to thermostat; control board shows thermostat input fault.

Possible Causes