Lennox E291 Error Code

What does Lennox E291 mean?

E291 triggers when the SL280DFNV detects that heat mode airflow has dropped below the minimum required for safe heat exchanger operation. The variable-speed blower reports its actual CFM to the IFC; when airflow falls below the minimum threshold during a gas heat call, the board logs E291 to warn of a restriction condition that will eventually trip the high-limit switch if unaddressed.

Symptoms

• E291 alert during a heating cycle — heat is produced but supply airflow is noticeably reduced.
• Discharge air temperature is high; temperature rise approaches or exceeds the rated maximum.
• High-limit switch trips on subsequent heating cycles.
• Blower runs but airflow at registers is weaker than normal.

Common causes

• Clogged or severely restricted air filter reducing system airflow below the minimum heat mode requirement.
• Closed or blocked supply or return air registers increasing system static pressure beyond the blower's operating range.
• Blower wheel accumulation of dirt or lint reducing effective impeller diameter and airflow.
• Duct system static pressure too high for the configured blower speed, limiting achievable airflow.
• ECM blower motor fault causing it to run below the commanded speed.

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Diagnostic steps

1. Replace the air filter immediately

   Install a clean filter of the correct MERV rating for the system. A clogged filter is the leading cause of E291 and the simplest fix. Verify the filter rack is fully sealed with no bypass air paths.

2. Verify all supply and return registers are open

   Walk the entire building and confirm every supply and return register is fully open. Closed registers raise static pressure and prevent the blower from achieving minimum heat airflow.

3. Inspect and clean the blower wheel

   Remove the blower assembly and inspect the wheel for lint, dirt, or debris accumulation on the blades. A dirty wheel significantly reduces airflow even with the motor running at the correct speed. Clean with a stiff brush.

4. Measure total external static pressure

   Using a manometer, measure static pressure at the return air plenum and supply air plenum. Total external static pressure above the SL280DFNV rated maximum for the configured blower speed confirms a duct restriction.

5. Verify ECM blower motor speed in diagnostic mode

   Use the IFC diagnostic mode to check actual reported blower RPM against the commanded speed. A significant shortfall indicates a motor module or motor fault requiring replacement.

When to call a professional

Static pressure measurement and ECM blower speed verification require a manometer and knowledge of the system design parameters. A licensed technician can measure actual system airflow using flow measurement equipment, compare it against the SL280DFNV rated minimum, and determine whether the restriction is in the duct system, blower wheel, or motor — and make the correct correction.