Lennox SLP98UHV Error Code E 291: Restricted Air Flow

What Does Code E 291 Mean?

Error code E291 on the Lennox SLP98UHV indicates that airflow (measured in cubic feet per minute) has dropped below what is needed for the minimum firing rate. The SLP98UHV's variable-capacity ECM blower motor monitors its own airflow by tracking motor torque and speed. When actual airflow falls significantly below the required level, the control board generates E291.

This is a serious airflow restriction alert. Unlike a simple limit switch trip that only detects overheating, E291 means the control board has measured airflow and determined it is insufficient for safe combustion. The furnace may reduce its firing rate or shut down entirely to prevent damage.

The most common cause is a dirty or clogged air filter. As the filter accumulates dust and debris, it creates resistance that even the variable-speed blower cannot overcome. Other common causes include closed or blocked supply registers and return grilles. Less common causes include a dirty blower wheel, collapsed ductwork, or a failing blower motor. If the restriction is not corrected, E291 will continue to appear on every heating cycle.

Common Causes

How to Fix It: Replace the Air Filter and Clear Airflow Restrictions

What You'll Need

• Replacement air filter (matching size) 🛒 Find at FiltersFast · 🛒 Find at Amazon
• Flashlight

Steps

1. Turn off electrical power at the breaker and shut off the gas supply valve Locate the furnace circuit breaker and flip it to OFF. Turn the gas shutoff valve to the OFF position (handle perpendicular to the pipe). If you smell gas, leave immediately and call your gas company.
2. Replace the air filter Locate the air filter in the blower compartment or return air duct. Remove the old filter and replace it with a new one of the correct size (printed on the old filter frame). Make sure the airflow direction arrow on the new filter points toward the blower. If you recently upgraded to a higher-MERV filter, the increased resistance may be triggering E291 — consider returning to the originally specified MERV rating.
3. Open all supply registers and return grilles Walk through every room and ensure all supply registers are open and not blocked by furniture, rugs, or curtains. Check all return air grilles as well. On a variable-capacity furnace like the SLP98UHV, even a few closed registers can push airflow below the minimum threshold.
4. Inspect accessible ductwork If you can see your ductwork in the basement, attic, or crawl space, check for kinked flex duct, disconnected joints, or crushed sections. Major duct restrictions directly reduce the airflow the blower can deliver.
5. Check the area around the furnace return Make sure nothing is stored against or blocking the return air path leading to the furnace. Items stacked near the return duct or furnace intake can create a significant restriction.
6. Restore power and gas, then test Turn the gas supply valve to ON (handle parallel to the pipe) and flip the circuit breaker to ON. Set your thermostat to call for heat and monitor the furnace through a complete heating cycle.

When to Call a Professional

Contact a licensed HVAC technician if:

• E291 persists after replacing the filter and opening all vents
• The blower motor sounds normal but airflow from registers is weak
• The blower wheel is visibly coated with dust and debris
• You suspect the duct system is undersized for the furnace
• E291 appears together with E250 or E311 (multiple airflow-related codes)

✓ Verified against manufacturer service manual — March 2026