Lennox E250 Error Code

What does Lennox E250 mean?

E250 indicates the primary limit switch circuit is open on a Lennox communicating furnace. The limit switch is a thermal safety device that opens when the heat exchanger gets too hot. If the switch does not close within three minutes, the furnace enters a 60-minute soft lockout (Watchguard mode). The root cause is almost always insufficient supply airflow or an overfired condition.

Symptoms

• Furnace shuts off before completing a heat cycle.
• E250 or 'GF Primary Limit Switch Open' notification on the Lennox thermostat.
• Blower continues to run after burners shut off as the control tries to cool the heat exchanger.
• System repeats short run cycles before entering soft lockout.

Common causes

• Dirty or blocked air filter reducing airflow across the heat exchanger.
• Closed or obstructed supply/return registers.
• Malfunctioning zone dampers holding too many zones closed simultaneously.
• High gas pressure causing the furnace to fire above rated capacity.
• Dirty blower wheel reducing CFM output.

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Diagnostic steps

1. Check and replace the air filter

   A severely restricted filter is the most common trigger for E250. Inspect the filter and replace it if it has any visible loading. Ensure the filter door or access panel seals correctly so air is not bypassing the filter.

2. Confirm all supply and return registers are open

   Open all registers fully, including those in unused rooms. Closed registers cause static pressure to rise and airflow to fall, directly overheating the heat exchanger.

3. Check zone damper positions

   In Lennox Smart Zoning systems, a limit trip forces the system to central mode. Verify each damper actuator opens correctly on a heat call and that the minimum-open-duct-area requirement is met.

4. Measure temperature rise across the heat exchanger

   With the furnace running, measure supply-air temperature minus return-air temperature. Compare to the nameplate rise range. A rise above the maximum indicates low airflow or overfiring.

5. Verify gas manifold pressure

   Connect a manometer to the manifold tap and confirm pressure matches the specification on the rating plate. High gas pressure increases firing rate beyond design limits.

When to call a professional

Engage a licensed technician if the limit trips again after filter and register corrections, if the temperature rise exceeds nameplate limits, or if the limit switch itself tests open at ambient temperature (indicating a failed switch). A cracked heat exchanger can mimic airflow-related limit trips and must be ruled out with combustion analysis — a task requiring professional equipment.