Error Code Limit Switch Circuit Open

Lennox SLP98UHV Error Code Limit Switch Circuit Open: Limit Switch Circuit Open

TL;DR

Your Lennox furnace's high-temperature limit switch tripped because the heat exchanger got too hot, almost always due to restricted airflow from a dirty filter or blocked vents. Replace the filter and check that all supply and return vents are open.

Disclaimer

This article is for informational purposes only and does not guarantee the accuracy or completeness of the information provided. Always turn off power and gas supply before attempting any repairs. If you smell gas, leave immediately and call your gas company. Consult a licensed HVAC technician for diagnosis and repair. Any actions taken based on this information are at your own risk.

What Does Code Limit Switch Circuit Open Mean?

Error code E250 on the Lennox IFC control board means the high-temperature limit switch circuit is open. The limit switch is a safety device that shuts down the burners when the air inside the furnace exceeds a safe temperature — typically around 180–200°F depending on the model. When the heat exchanger overheats, the limit switch opens the circuit to prevent damage to the furnace or a potential fire hazard.

The number one reason a limit switch trips is restricted airflow. When the blower cannot move enough air across the heat exchanger, heat builds up until the limit switch intervenes. The most common airflow restriction is a dirty or clogged air filter, followed by blocked or closed supply/return registers in the home. A dirty blower wheel (caked with dust) can also reduce airflow enough to cause overheating.

Once the limit switch trips and the furnace cools down, the switch will reset itself (it's an automatic reset device on most Lennox models). However, if the root cause isn't fixed, the furnace will keep cycling: burners on → overheat → limit trips → cool down → burners on again. This repeated cycling will eventually escalate to E274 (limit circuit recycles exceeded) and the furnace will lock out.

How to Fix It: Replace the Air Filter and Check Vents

⚠ Safety First

Always turn off the furnace at the power switch or breaker and shut off the gas supply before beginning. Do not proceed if you smell gas — leave the area and call your gas company immediately.

What You'll Need

New furnace air filter (correct size for your Lennox unit)
Flashlight

Steps

Turn off power AND shut off gas supply Switch the furnace power off at the disconnect switch near the unit AND at the circuit breaker. Locate the gas shutoff valve on the gas line leading to the furnace and turn it to the OFF position (perpendicular to the pipe). If you smell gas at any point, leave the area immediately and call your gas company from outside.
Locate and remove the air filter Find the air filter — on most Lennox furnaces, it is located in the return air duct opening on the side or bottom of the furnace cabinet, or in a slot between the return duct and the blower compartment. Slide the old filter out and note the size printed on the filter frame.
Inspect the old filter Hold the old filter up to a light. If you can barely see light through it, or if it is visibly caked with dust and debris, it was restricting airflow and likely caused the overheating. Even if the filter doesn't look terrible, replace it if it has been in use for more than 90 days (30 days for 1-inch filters).
Install the new filter Insert the new filter into the filter slot, making sure the airflow direction arrow on the filter frame points toward the furnace (toward the blower). Ensure the filter fits snugly with no gaps around the edges where air could bypass it.
Check all supply and return vents throughout the home Walk through every room and make sure all supply registers (where warm air comes out) and return air grilles (where air goes back to the furnace) are fully open and not blocked by furniture, rugs, curtains, or other objects. At least 80% of registers should be open for adequate airflow. On Lennox systems, closing too many registers forces the furnace to work harder with less airflow.
Inspect the blower compartment (optional) While the power is off, open the blower compartment door and use a flashlight to check the blower wheel. If the wheel blades are caked with dust and debris, this can significantly reduce airflow. You can vacuum loose debris, but thorough blower cleaning may require a technician.
Restore power and test Replace the access panel. Turn the gas supply back on (valve handle parallel to the pipe). Restore power at the disconnect switch and breaker. The limit switch should have reset itself while the furnace was off and cool. Set your thermostat to call for heat.

How to Verify

After restoring power and calling for heat, the furnace should run through a complete heating cycle without the E250 code returning. Feel the supply register airflow — it should be strong and steady. If the furnace runs for 10+ minutes and the limit switch trips again (E250 reappears), the problem may be a failing blower motor or a blower wheel that needs professional cleaning.

When to Call a Professional

Contact a licensed HVAC technician if:

The filter is clean and all vents are open but E250 continues to occur
The blower motor sounds unusually loud, makes grinding noises, or runs slowly
The blower wheel is heavily coated with dust and debris
The furnace trips the limit switch within minutes of starting, even with a new filter
You notice a gas smell at any point during the process