Heil furnace LED flashes 6 times; furnace may not respond to thermostat or may run erratically.

Safety: This involves line-voltage wiring. If you are not experienced with electrical work, contact a qualified electrician or HVAC technician.

Steps to check:

• Verify power source: Turn off the breaker feeding the Heil furnace. Remove the electrical junction box cover on the furnace. Confirm that hot (usually black) is connected to L1, neutral (white) to neutral, and ground (green/bare) to the ground screw.
• Check receptacle (if cord-connected): Use a plug-in outlet tester to verify correct polarity and ground at the receptacle.
• Inspect ground connection: Ensure the furnace chassis is bonded to the electrical ground and that the ground wire is tight and not corroded.
• Reset power: After correcting any wiring issues, restore power and see if the error clears.

When to call a pro: If wiring appears correct but the board still reports a polarity/ground fault, the control board may be defective or there may be a more complex grounding issue. Have a professional diagnose and replace the control board if necessary.