What does Lennox Furnace Blowing Cold Air - Causes & Fix mean?

Usually caused by thermostat fan set to ON instead of AUTO, or a dirty filter blocking airflow. Switch fan to AUTO and replace filter.

How do I fix Lennox Furnace Blowing Cold Air - Causes & Fix?

Follow the step-by-step diagnosis below. The most likely fix is identified by checking power, sensor resistance, and control-board signals in order — using a multimeter and the part numbers documented in the parts reference table.

Can I keep running the equipment with this fault active?

Generally no. Faults on Hvac units are emitted when a sensor, safety, or control circuit is outside its operating envelope. Running through an active fault risks compressor, heater, or control-board damage. Diagnose and clear the fault first.

What part is most commonly replaced for Lennox Furnace Blowing Cold Air - Causes & Fix?

The most frequently replaced parts for Lennox Furnace Blowing Cold Air - Causes & Fix are listed in the Parts Reference section of this guide with OEM part numbers and typical price ranges. Check the table for direct replacements.

Is Lennox Furnace Blowing Cold Air - Causes & Fix a DIY repair or do I need a technician?

Sensor swaps, fan motor replacements, and condenser cleaning are typical DIY-level repairs. Anything involving refrigerant, gas, or high-voltage (>240V) work requires a licensed technician for both safety and code-compliance reasons.

Lennox Furnace Blowing Cold Air - Causes & Fix

Lennox Furnace Blowing Cold Air — What’s Happening

A Lennox furnace blowing cold air is a symptom, not a fault code. It means your system is moving air through the house but the burner is either not lighting, not staying lit, or the heated air is not reaching your rooms. Lennox notes that a short burst of cool air at startup is normal, but continuous cold air indicates a real problem.

The blower may be running all the time even when the furnace is not calling for heat, or the heating cycle may be starting but failing before warm air is delivered. The most common causes are thermostat settings (fan stuck on ON), restricted airflow from a clogged filter, ignition or flame-sensor faults that prevent burners from staying lit, blocked condensate drains on high-efficiency models, gas-supply or gas-valve issues, and duct leaks that lose heat before it reaches the living space.

Jump to Fix

Most Likely Causes

Thermostat fan set to ON instead of AUTO When the fan switch is set to ON, the blower runs continuously even between heating cycles, pushing unheated air through the house.
Dirty or clogged air filter A restricted filter blocks airflow and can make the furnace appear to blow only cold air or prevent proper heating performance.
Ignition or flame-sensor problems Lennox notes that pilot or flame-sensor issues can stop burners from lighting or staying lit, so the blower runs but no heat is produced.
Blocked condensate drain (high-efficiency models) A clogged PVC drain line can shut down the furnace or prevent normal operation on condensing Lennox units.
Gas supply or gas-valve fault Lennox identifies a faulty gas valve as a cause when the pilot will not light or gas is not reaching the burners.
Duct leakage or disconnected ductwork Heated air may be lost through holes or disconnected runs before it reaches the rooms, making the house feel cold even if the furnace is heating.

How to Diagnose and Fix

Confirm whether the blower is running with no heat at all, or whether you only feel a brief cool purge at startup (which Lennox says is normal).
Check the thermostat mode and fan settings: set mode to Heat and fan to AUTO, and replace thermostat batteries if low.
Inspect and replace the air filter if dirty or clogged, and confirm that supply registers and return grilles are not blocked.
Watch the ignition sequence at the furnace: verify whether the pilot or hot-surface igniter starts, whether burners light, and whether the flame stays on.
Check the flame sensor and flame-proving circuit if burners light briefly then drop out (common service path consistent with Lennox guidance on pilot and flame-sensor issues).
Inspect the condensate drain line on condensing models: clear any blockage in the PVC drain path or condensate trap.
Verify gas supply and gas-valve operation (this step is best handled by a qualified technician).
Inspect accessible ductwork for leaks, holes, or disconnected sections if the furnace is heating but rooms stay cold, then cycle power at the breaker and re-test.

Parts You Might Need

Part	Notes
Air filter (size stamped on old filter frame)	Amazon \| Replace every 1–3 months depending on household dust and pets.
Flame sensor	Amazon \| Often needs cleaning or replacement if burners light then shut off immediately.
Thermostat batteries (if applicable)	Amazon \| Low batteries can cause erratic fan or mode behavior.

If your appliance also shows a code on the display, these match this problem:

When to Call a Pro

Call a qualified HVAC technician if the burners will not light, if burners light then drop out after you have checked the filter and thermostat settings, if you suspect a gas-valve or gas-supply problem, or if you see a diagnostic fault code on the furnace control board. Gas-related work, ignition component testing, flame-sensor cleaning or replacement, and refrigerant work (on heat pumps) all require proper tools and safety training. If you have cleared the filter and confirmed AUTO fan mode but still get continuous cold air, a pro can read the exact fault code and follow the model-specific service manual to pinpoint the cause. For gas line, burner, or igniter work, or if you ever smell gas, stop and call a licensed technician.