Lennox SLP98UHV Error Code E 250: Limit Switch Circuit Open
What Does Code E 250 Mean?
On the Lennox SLP98UHV, E 250 means the limit switch circuit is open — the furnace's high-temperature safety limit has sensed excessive heat and shut off the burners. This protects the heat exchanger from overheating, which could crack it or create a fire hazard. When the limit opens, the burners cut out but the ECM blower keeps running to purge heat from the exchanger.
E 250 sits at the head of this board's airflow-fault family. If the limit keeps tripping, the SLP98UHV escalates to E 274, a soft lockout after repeated limit trips. The same underlying low-airflow condition also drives E 252 (discharge air temperature too high), E 291 (measured airflow below the minimum firing rate), E 311 (heat rate cut back because of low airflow), and E 312 (airflow cutback in cooling or fan mode). Across all of them, a clogged air filter is the usual root cause.
Because the SLP98UHV is a modulating furnace, it needs a steady volume of return air across the heat exchanger at whatever rate it is firing. The most common homeowner-fixable cause of E 250 is a dirty air filter starving that airflow; closed or blocked supply and return registers do the same thing. Deeper causes — an overfired gas condition, a weak blower, or undersized ductwork — are for a technician. The limit resets by itself once the furnace cools to a safe temperature.
What You'll Notice
- The burners light but shut off after only a short time, producing little heat
- The blower keeps running (often on high) after the burners have gone out
- The home is not reaching the thermostat setpoint despite the furnace cycling
- The 7-segment display shows E 250 when you recall codes
- The air filter looks gray, matted, or clogged when you pull it
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 This Is Diagnosed
The airflow path is checked from the easiest restriction to the hardest: first the filter, then the supply and return registers, then visible ductwork. If airflow is clearly adequate and the limit still trips, a technician measures temperature rise and gas input to rule out an overfired condition or a failing blower.
How to Fix It: Replace the Air Filter and Restore Airflow
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). Let the furnace cool before working near it. 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 pull it. If it is dirty or clogged it is very likely the cause. Install a new filter of the correct size with the airflow arrow pointing toward the blower. If you recently switched to a high-MERV filter, its added resistance can trip the limit — consider returning to the MERV rating Lennox recommends.
- Open every supply register and return grille Walk the house and make sure all supply registers and return grilles are fully open and not covered by furniture, rugs, or curtains. On a modulating furnace, several closed registers can drop airflow enough to trip the limit.
- Inspect accessible ductwork Look at any exposed ducting in the basement, attic, or crawl space for collapsed flex duct, disconnected joints, or crushed sections that choke airflow.
- Restore power and gas, then test Turn the gas valve to ON (handle parallel to the pipe) and switch the breaker to ON — cycling power also clears any lockout. Set the thermostat to call for heat and watch a full cycle.
When to Call a Professional
Contact a licensed HVAC technician if:
- E 250 returns after replacing the filter and opening all vents
- The blower sounds labored or clearly is not spinning at full speed
- The limit trips within a few minutes of every furnace start
- E 274 also appears, meaning the repeated tripping has caused a soft lockout
- You suspect the duct system is undersized for the furnace's capacity
Frequently Asked Questions
Can I just reset the furnace and keep using it?
A reset may buy one more cycle, but if the airflow restriction is still there the limit will trip again and can escalate into an E 274 soft lockout. Fix the airflow cause before relying on the furnace.
How often should I change the filter to prevent E 250?
It varies by home, filter type, pets, and how hard the system runs, but many homeowners change it every one to three months. Check it monthly during heating season and replace it once it looks loaded.
The filter is clean but I still get E 250 — why?
Other airflow restrictions such as closed vents, a dirty blower wheel, or undersized ductwork can trip the limit, and so can an overfired gas condition. Those causes are outside DIY scope and need a technician.
✓ Verified against manufacturer service manual — March 2026