Lennox SLP98UHV Error Code E 291: Restricted Air Flow
What Does Code E 291 Mean?
On the Lennox SLP98UHV, E 291 means the airflow (in cubic feet per minute) has fallen below what the furnace needs to support even its minimum firing rate. This is not a simple over-temperature trip — the variable-speed ECM blower on this furnace infers its own airflow from motor torque and speed, so E 291 means the SureLight control has effectively measured airflow and found it insufficient for safe combustion.
E 291 is one of the most direct airflow faults in this board's family. It is closely tied to E 311, where the furnace cuts back its heat rate to match reduced airflow, and E 312, where the blower cuts back in cooling or fan mode. It also shares its root cause with the overheating codes: E 250 (limit switch open), E 274 (soft lockout after repeated limit trips), and E 252 (discharge air too hot). Where those codes react to the heat that low airflow produces, E 291 flags the low airflow itself, often before the furnace overheats.
The most common homeowner-fixable cause is a dirty or clogged air filter creating more resistance than even the variable-speed blower can overcome. Closed or blocked supply registers and return grilles do the same thing. Less common causes — a dust-caked blower wheel, collapsed ductwork, or a failing blower motor — need a technician. Until the restriction is cleared, E 291 tends to reappear on every heating cycle.
What You'll Notice
- Airflow from the supply registers feels weak even though the blower is running
- The furnace reduces output or shuts down instead of running normally
- The 7-segment display shows E 291 when you recall codes
- The air filter is visibly dirty, gray, or clogged
- One or more supply registers or returns are closed or blocked by furniture
Common Causes
How This Is Diagnosed
The restriction is traced from the most common and accessible point inward: the filter first, then the supply and return registers, then the return-air path and visible ductwork. If airflow is still short with a clean filter and open vents, a technician inspects the blower wheel, motor, and duct sizing.
How to Fix It: Replace the Air Filter and Clear Airflow Restrictions
What You'll Need
- Replacement air filter (matching size printed on the old frame) 🛒 Find at FiltersFast · 🛒 Find at Amazon
- Flashlight
Steps
- Turn off electrical power at the breaker and shut off the gas supply valve Switch the furnace breaker to OFF and turn the manual gas shutoff valve to the OFF position (handle perpendicular to the pipe). If you smell gas, leave immediately and call your gas company.
- Replace the air filter Find the filter in the blower compartment or return duct and replace it with a new one of the correct size (printed on the old frame), airflow arrow pointing toward the blower. If you recently upgraded to a high-MERV filter, its added resistance may be triggering E 291 — consider returning to the MERV rating Lennox specifies.
- Open every supply register and return grille Walk the house and confirm all supply registers are open and not blocked by furniture, rugs, or curtains, and check the return grilles too. On a modulating furnace, even a handful of closed registers can push airflow below the minimum threshold.
- Inspect accessible ductwork Where you can see ducting in the basement, attic, or crawl space, check for kinked flex duct, disconnected joints, or crushed sections that directly cut the airflow the blower can deliver.
- Clear the area around the return Make sure nothing is stacked against or covering the return-air path to the furnace. Items piled near the return duct or intake can create a major restriction on their own.
- Restore power and gas, then test Turn the gas valve to ON (handle parallel to the pipe) and switch the breaker to ON. Set the thermostat to call for heat and monitor a full heating cycle.
When to Call a Professional
Contact a licensed HVAC technician if:
- E 291 persists after replacing the filter and opening all vents
- The blower motor sounds normal but airflow at the registers stays weak
- The blower wheel is visibly caked with dust and debris
- You suspect the duct system is undersized for the furnace
- E 291 appears together with E 250 or E 311, pointing to a broad airflow problem
Frequently Asked Questions
How is E 291 different from the limit-switch code E 250?
E 250 trips when the furnace has already overheated, while E 291 flags the low airflow itself — the board measures airflow through the ECM blower and sees it fall below the minimum. They share the same usual cause, a dirty filter, but E 291 often shows up before an overheat trip.
Can closing vents in unused rooms cause E 291?
Yes. Closing off registers raises static pressure and lowers total airflow, and on a modulating furnace that can be enough to drop below the minimum and trigger E 291. Open the vents back up and see if the code clears.
I replaced the filter and E 291 is gone — do I still need a technician?
If a fresh filter and open vents clear it and it stays gone, no. If it keeps returning, have a technician check the blower wheel, motor, and duct sizing, since those causes are outside safe DIY scope.
✓ Verified against manufacturer service manual — March 2026