Heil furnace LED flashes 9 times; furnace may not respond to thermostat or may behave erratically.

Safety: Turn off power before moving any thermostat or control wires.

Steps to check:

• Verify thermostat wiring: At the furnace control board, confirm that the thermostat R wire is on R, C on C, W on W, Y on Y, and G on G. Match these at the thermostat sub-base.
• Check external connections: If your Heil furnace is connected to an outdoor condenser or heat pump, verify that the Y and C wires are correctly landed at both ends and not swapped.
• Look for shared transformer issues: If multiple systems share a common 24V transformer, ensure they are properly phased and not backfeeding each other. This is typically a technician-level task.
• Reset control: After correcting wiring, power cycle the furnace and see if the error clears.

When to call a pro: If wiring appears correct but the error persists, the control board may be misinterpreting signals and require replacement by a professional.