Lennox Furnace 9 flashes (older boards): What It Means & How to Fix It
Compiled & reviewed by the HVAC Flash Codes editorial team · Updated 2026-06-25 · confidence: high
⚠️ Try the safe checks, then a pro
What does Lennox furnace 9 flashes (older boards) mean?
On older red-LED Lennox controls, 9 flashes = the pressure switch failed to close, or opened during a heat demand — the combustion-air/venting pressure switch did not prove the inducer's draft. Most often caused by Blocked or restricted intake/exhaust venting.
What your furnace LED does. The diagnostic light blinks 9 flashes, pause, repeat — that pattern is Lennox furnace 9 flashes (older boards).
What Lennox furnace 9 flashes (older boards) means
On older red-LED Lennox controls, 9 flashes = the pressure switch failed to close, or opened during a heat demand — the combustion-air/venting pressure switch did not prove the inducer's draft.
Common causes
Blocked or restricted intake/exhaust venting
Plugged condensate drain or cold-end heater box
Cracked or disconnected pressure-switch hose
A weak/failed inducer or a faulty pressure switch
Homeowner-safe steps to try
Set the thermostat to OFF
Clear the outside intake/exhaust terminations (ice, snow, debris, nests)
Confirm the condensate drain is clear and draining
Replace the air filter and cycle power to retry
When to call a licensed HVAC technician
If the basic checks below don't clear the code, the fault recurs, or anything involves the gas valve, burners, flame sensor or wiring, stop and book a licensed HVAC technician. Repairs to the combustion or gas system are not DIY.
⚠️ Safety first. Gas furnaces burn fuel and produce combustion gases. If you ever smell gas (a rotten-egg or sulfur odor), hear hissing, or your carbon-monoxide alarm sounds, do not touch light switches, thermostats, or the furnace — leave the home immediately and call your gas utility's emergency line or 911 from outside. Homeowners should limit themselves to simple, safe steps: setting the thermostat, checking and replacing the air filter, confirming the furnace switch and breaker are on, keeping supply registers and return grilles open, and clearing snow, ice, or debris from outdoor vent and intake terminations. Never bypass, jumper, or reset safety switches (limit, flame-rollout, or pressure switches), never open the gas valve or burner compartment, and never attempt combustion, electrical, or control-board repairs yourself. Error codes point toward a problem but are not a substitute for proper diagnosis — when in doubt, turn the system off and call a licensed HVAC technician. Keep working carbon-monoxide detectors on every level of your home.
When to call a pro: Call a licensed technician if it persists after venting and drain are clear — the inducer, hose, or pressure switch may need service. Do not bypass the pressure switch. If you smell gas, leave and call the gas utility from outside.
Lennox furnace 9 flashes (older boards) FAQ
What does Lennox furnace 9 flashes (older boards) mean?
On older red-LED Lennox controls, 9 flashes = the pressure switch failed to close, or opened during a heat demand — the combustion-air/venting pressure switch did not prove the inducer's draft.
What causes Lennox furnace 9 flashes (older boards)?
Common causes: Blocked or restricted intake/exhaust venting; Plugged condensate drain or cold-end heater box; Cracked or disconnected pressure-switch hose; A weak/failed inducer or a faulty pressure switch.
Can I fix Lennox furnace 9 flashes (older boards) myself?
You can safely try: Set the thermostat to OFF; Clear the outside intake/exhaust terminations (ice, snow, debris, nests); Confirm the condensate drain is clear and draining; Replace the air filter and cycle power to retry. Call a licensed technician if it persists after venting and drain are clear — the inducer, hose, or pressure switch may need service. Do not bypass the pressure switch. If you smell gas, leave and call the gas utility from outside.