Furnace does not respond to thermostat; LED shows a slow continuous flash with no distinct count.

Safety: Turn off power before working on thermostat wiring.

Steps:

• Check thermostat mode: Ensure thermostat is set to HEAT and the setpoint is above room temperature. Replace batteries if applicable.
• Verify 24 VAC at R and C: At the furnace control board, measure between R and C terminals. You should see ~24 VAC. If not, check the low-voltage fuse on the board and replace with the same rating if blown.
• Check W call: With a call for heat, measure between R and W. You should see 24 VAC. If not, the thermostat or wiring may be open. Temporarily jumper R to W at the furnace board to see if the furnace starts; if it does, the issue is upstream (thermostat/wiring).
• Inspect thermostat cable: Look for damage where the thermostat cable runs along walls or near the furnace. Repair or replace damaged sections.

If the furnace responds to an R-W jumper but not to the thermostat, replace or reprogram the thermostat according to the manufacturer's instructions.