Lennox SLP98UHV Error Code E 252: Discharge Air Temperature Too High
What Does Code E 252 Mean?
Error code E252 on the Lennox SLP98UHV indicates the discharge air temperature is too high during gas heat operation. The furnace has a sensor that monitors the temperature of air leaving the furnace, and it has exceeded the acceptable range.
This code is closely related to E250 (limit switch open) — both indicate overheating, but E252 specifically monitors the discharge air temperature rather than the internal furnace temperature. The alert clears when the current heating cycle finishes.
The most common cause is restricted airflow due to a dirty air filter or blocked ductwork. When the furnace cannot push enough air across the heat exchanger, the air that does pass through gets heated to an excessively high temperature. Other causes include an incorrect gas input rate (overfired furnace) or a blower motor running at too low a speed.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Low airflow due to dirty filter or restricted ductwork | Most common | ✓ DIY fix → |
How to Fix It: Replace the Air Filter and Restore Airflow
What You'll Need
- Replacement air filter (matching size) 🛒 Find at FiltersFast · 🛒 Find at Amazon
- Flashlight
Steps
- Turn off electrical power and gas supply Turn off the furnace at the circuit breaker and turn the gas shutoff valve to the OFF position (handle perpendicular to the pipe). Allow the furnace to cool.
- Replace the air filter Locate the air filter in the blower compartment or return air duct. If the filter is dirty or clogged, replace it with a new filter of the correct size. Make sure the airflow arrow on the filter points toward the blower.
- Open all supply registers and return grilles Walk through every room and ensure all supply registers and return grilles are fully open and unobstructed by furniture, rugs, or curtains.
- Check for ductwork restrictions Inspect accessible ductwork for collapsed flex duct, disconnected sections, or crushed runs that could restrict airflow.
- 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 let the furnace complete a full cycle.
When to Call a Professional
Contact a licensed HVAC technician if:
- E252 returns after replacing the filter and opening all vents
- You also see E250 or E274 codes (limit switch and lockout)
- The air from the registers feels unusually hot compared to normal
- The blower motor sounds labored or seems to be running slowly
- The gas input rate may need to be adjusted by a technician
✓ Verified against manufacturer service manual — March 2026