Lennox SLP98UHV Error Code E 252: Discharge Air Temperature Too High

What Does Code E 252 Mean?

On the Lennox SLP98UHV, E 252 means the discharge air temperature — the temperature of the air leaving the furnace during gas heat — has climbed too high. The SureLight control reads this from the discharge air temperature sensor and flags E 252 when the reading exceeds the safe range for the current firing rate.

E 252 is a close cousin of E 250 (limit switch open); both are overheating faults, but E 252 watches the temperature of the discharge air specifically, while E 250 watches the safety limit inside the furnace. It also belongs to the same airflow family as E 291 (measured airflow below the minimum firing rate), E 311 (heat rate cut back from low airflow), E 312 (airflow cutback in cooling or fan mode), and E 274 (soft lockout after repeated limit trips). When airflow drops, the air that does move across the heat exchanger picks up too much heat, and E 252 is often the first warning you see.

The most common homeowner-fixable cause is a dirty air filter or otherwise restricted airflow, so the heat exchanger cannot shed its heat into a normal volume of moving air. Because the SLP98UHV modulates its output, an airflow shortfall shows up quickly as an over-temperature discharge. Deeper causes — an incorrect gas input rate (an overfired furnace) or a blower running too slowly — require a technician to measure temperature rise and input. The alert clears when the cycle finishes with the temperature back in range.

What You'll Notice

• Air from the supply registers feels unusually, uncomfortably hot
• The furnace runs in short bursts and cuts out before satisfying the thermostat
• The 7-segment display shows E 252 when you recall codes
• The air filter is visibly dirty or clogged
• E 250 or E 274 may show up alongside E 252

Common Causes

How This Is Diagnosed

Airflow is checked first because it is both the most common cause and the only homeowner-safe one: the filter, then the registers, then visible ducting. If airflow is confirmed good and the discharge is still too hot, a technician measures the temperature rise and gas input rate to find an overfired or blower-related cause.

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 Switch the furnace breaker to OFF and turn the manual gas shutoff valve to the OFF position (handle perpendicular to the pipe), then let the furnace cool. If you smell gas, leave immediately and call your gas company.
2. Replace the air filter Locate the filter in the blower compartment or return duct. If it is dirty or clogged, replace it with a new filter of the correct size, with the airflow arrow pointing toward the blower. A high-MERV filter that adds resistance can also drive the discharge temperature up.
3. Open every supply register and return grille Go room to room and make sure all supply registers and return grilles are fully open and not blocked by furniture, rugs, or curtains, so the furnace has a full path for return and supply air.
4. Check accessible ductwork for restrictions Inspect any exposed ducting for collapsed flex duct, disconnected sections, or crushed runs that would limit airflow across the heat exchanger.
5. 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 let the furnace run a full cycle.

When to Call a Professional

Contact a licensed HVAC technician if:

• E 252 returns after replacing the filter and opening all vents
• E 250 or E 274 shows up along with E 252
• The register air stays much hotter than it normally feels
• The blower sounds labored or seems to run slowly
• The gas input rate or temperature rise may be out of spec and need adjustment

✓ Verified against manufacturer service manual — March 2026