Lennox SLP98UHV Error Code E 250: Limit Switch Circuit Open
What Does Code E 250 Mean?
Error code E250 on the Lennox SLP98UHV indicates that the limit switch circuit is open. The limit switch is a safety device that shuts down the burners when the furnace's internal temperature exceeds a safe threshold. This prevents the heat exchanger from overheating, which could crack it or create a fire hazard.
When the limit switch opens, the burners turn off but the blower continues running to cool the heat exchanger. If the limit switch does not close within 3 minutes, the furnace enters a 1-hour soft lockout (E274). The limit switch resets automatically once the temperature drops to a safe level.
The most common cause is a dirty air filter restricting airflow across the heat exchanger. When insufficient air passes over the exchanger, heat builds up until the limit switch trips. Other causes include blocked or closed supply and return vents, an overfired condition (incorrect gas pressure), or a blower motor running at insufficient speed.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Dirty air filter restricting airflow and causing overheating | Most common | ✓ DIY fix → |
| Blocked or closed supply/return registers restricting airflow | 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 before working near it.
- Replace the air filter Locate the air filter in the blower compartment or return air duct. Remove the old filter and check its condition — if it is visibly dirty or clogged, it is likely the cause. Install a new filter of the correct size with the airflow direction arrow pointing toward the blower. If you recently upgraded to a higher-MERV filter, consider going back to the originally recommended MERV rating, as higher ratings create more air resistance.
- Open all supply registers and return grilles Walk through every room and make sure all supply registers and return air grilles are fully open and not blocked by furniture, rugs, curtains, or other objects.
- Inspect accessible ductwork Check visible ductwork in the basement, attic, or crawl space for collapsed flex duct, disconnected sections, or crushed portions.
- 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. Cycling the power resets any lockout. Set your thermostat to call for heat and monitor through a complete cycle.
When to Call a Professional
Contact a licensed HVAC technician if:
- E250 returns after replacing the filter and opening all vents
- The blower motor sounds labored or is not spinning at full speed
- The limit switch trips within minutes of the furnace starting
- You also see E274 (soft lockout from repeated limit trips)
- The duct system may be undersized for the furnace capacity
✓ Verified against manufacturer service manual — March 2026