Lennox SLP98UHV Error Code Restricted Airflow in Cooling/Continuous Fan Mode: Restricted Airflow in Cooling/Continuous Fan Mode

What Does Code Restricted Airflow in Cooling/Continuous Fan Mode Mean?

Error code E312 on the Lennox IFC control board indicates that airflow during cooling or continuous fan operation has been measured as restricted — below the minimum acceptable level. While this code is associated with cooling/fan operation rather than heating, the root cause is the same airflow restriction issue that triggers E291 and E311 during heating mode.

Lennox communicating furnaces with variable-speed blower motors continuously monitor airflow by tracking motor torque and RPM. During cooling mode (when the blower is moving air across the evaporator coil) or during fan-only mode, the board compares actual airflow to the configured minimum. If actual airflow falls short, E312 is generated.

The most common cause is a clogged air filter. Because the same filter and blower system handles both heating and cooling airflow, a dirty filter affects both modes. Other causes include blocked return or supply vents, a dirty evaporator coil (which the blower must push air through during cooling), or a dirty blower wheel. On some Lennox systems, this code can also appear if the cooling airflow CFM was configured too high for the ductwork during installation.

How to Fix It: Replace the Air Filter and Check Vents

What You'll Need

• New furnace air filter (correct size for your Lennox unit)
• Flashlight

Steps

1. Turn off power AND shut off gas supply Switch the furnace power off at the disconnect switch near the unit AND at the circuit breaker. Locate the gas shutoff valve on the gas line leading to the furnace and turn it to the OFF position (perpendicular to the pipe). If you smell gas at any point, leave the area immediately and call your gas company from outside.
2. Replace the air filter Locate the air filter on your Lennox furnace — typically in the return air duct opening on the side or bottom of the cabinet. Remove the old filter, note the size, and install a new filter with the airflow direction arrow pointing toward the furnace blower.
3. Check all supply and return vents Walk through every room in the home and ensure all supply registers and return air grilles are fully open and unobstructed. Remove any furniture, rugs, curtains, or objects blocking vents. Restricted vents reduce the total system airflow and can trigger E312 during cooling or fan operation.
4. Inspect the evaporator coil area (if accessible) If you can see the evaporator coil (typically located above the furnace in the plenum), check whether the coil surface is visibly dirty or clogged with dust. A dirty evaporator coil adds significant resistance to airflow during cooling mode. Note: do not attempt to clean the coil yourself — this requires a professional with proper coil cleaner and tools.
5. Restore power and test Replace the access panel. Turn the gas supply back on (valve handle parallel to the pipe). Restore power at the disconnect switch and breaker. Set your thermostat to fan-only mode or cooling mode and let the system run to verify airflow has improved.

When to Call a Professional

Contact a licensed HVAC technician if:

• The filter is new and all vents are open but E312 continues to appear
• The evaporator coil is visibly dirty or iced over
• The blower motor sounds unusually loud or is not reaching full speed
• The code appeared after a new system installation or ductwork modification
• You notice a gas smell at any point during the process