Furnace will not start; LED shows 8 flashes; thermostat appears to call for heat but nothing happens.

Safety: Only work on line-voltage wiring if you are qualified. Otherwise, call an electrician or HVAC technician.

Steps:

• Check breaker and switch: Verify the furnace circuit breaker in the electrical panel is ON and not tripped. Confirm the furnace service switch (usually on or near the furnace) is ON.
• Inspect power cord and junction box: With power off at the breaker, remove the furnace electrical junction box cover. Check for loose wire nuts, burned wires, or signs of overheating. Tighten connections as needed.
• Verify polarity and ground: Use a plug tester or multimeter to confirm hot, neutral, and ground are correctly wired. The furnace cabinet must be bonded to ground.
• Reset control board: After correcting any power issues, turn power off for 30 seconds, then back on to reset the board and clear the 8-flash code.

If power and wiring are correct and the code persists, the control board may be defective and should be replaced by a professional.