Lennox SLP98UHV Error Code Restricted Airflow: Restricted Airflow

What Does Code Restricted Airflow Mean?

Error code E291 on the Lennox IFC control board indicates the system has measured that airflow during a heating cycle has fallen below the minimum threshold required for safe operation. Lennox communicating furnaces with variable-speed blower motors can actually measure airflow in real time by monitoring motor torque and speed. When the measured airflow drops below the programmed minimum for the current heating stage, the board flags E291.

This is different from the limit switch codes (E250/E274) in that the board is proactively detecting the airflow problem before the furnace overheats, rather than reacting to an overheating event after the fact. It's an early warning that airflow is insufficient. However, if ignored, the furnace will eventually overheat and trigger the limit switch as well.

The most common cause is a clogged air filter that is choking off return air to the blower. Other causes include blocked or closed supply/return registers, a dirty blower wheel, or a restrictive duct system. On some Lennox models, this code may also appear after ductwork modifications or if the system was set up with incorrect airflow parameters. If the airflow restriction is severe enough, the furnace may escalate to E311 (heat rate reduced to match airflow).

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. Make sure the airflow direction arrow on the filter frame points toward the furnace blower. On high-efficiency Lennox systems, use the filter type recommended in your owner's manual — overly restrictive aftermarket filters can cause airflow issues.
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 the vents. On Lennox communicating systems, the blower adjusts speed to maintain target airflow — but if too many vents are blocked, even maximum blower speed may not be enough.
4. Check the return air duct for obstructions Look inside the return air duct near the furnace for any obvious obstructions — collapsed flex duct, debris, or a filter that was installed in the wrong location. Use a flashlight to check as far as you can see.
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 call for heat and let the furnace run through a complete heating cycle.

When to Call a Professional

Contact a licensed HVAC technician if:

• The filter is new and all vents are open but E291 continues to appear
• The blower seems to be running at high speed but airflow is still weak
• You recently had ductwork modified or added
• The code appeared after an HVAC system installation or equipment change
• You notice a gas smell at any point during the process