Lennox E274 Error Code

What does Lennox E274 mean?

E274 is a soft lockout on Lennox communicating furnaces. It fires when the primary limit switch opens and stays open for more than three minutes across multiple ignition attempts, exceeding the maximum retry count. The furnace enters Watchguard mode and suspends ignition until the root cause — typically overheating from restricted airflow — is resolved and the limit resets.

Symptoms

• Furnace shuts down mid-cycle and does not restart automatically.
• E274 or 'GF Primary Limit Switch Open' displayed on the thermostat or control board.
• No heat output despite thermostat demand remaining active.
• System enters a 60-minute soft lockout (Watchguard mode) after repeated limit trips.

Common causes

• Dirty or clogged air filter blocking return airflow.
• Blocked or closed supply/return registers restricting airflow through the heat exchanger.
• Malfunctioning zone dampers holding sections of ductwork closed.
• Overfired furnace due to incorrect gas pressure or wrong unit size code.
• Restricted condensate drain or blocked cold-end heater box on 90%+ models.

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Diagnostic steps

1. Inspect and replace the air filter

   Remove the filter and hold it to light — replace if airflow is visibly obstructed. A clogged filter is the single most common cause of limit trips on this platform.

2. Check all supply and return registers

   Walk the home and confirm every register is fully open. Closed registers force the blower to push air through a smaller cross-section, raising heat-exchanger temperature.

3. Verify zone damper operation

   In iHarmony or Lennox Smart Zoning systems, check each damper actuator for correct response to a call. A stuck-closed damper starves the furnace of airflow. The zoning system should drop to central mode when the limit trips.

4. Measure gas manifold pressure and firing rate

   Connect a manometer to the gas valve manifold port and compare the reading to the nameplate. Overfiring raises heat-exchanger temperature independently of airflow.

5. Inspect the condensate drain and cold-end heater box

   On condensing models, clear any blockage in the drain strainers and outlet of the cold-end heater box. Blocked drainage can force the limit to trip even with adequate airflow.

6. Reset and confirm Watchguard mode exits cleanly

   After correcting the root cause, cycle power or wait for the 60-minute lockout to expire. Confirm the furnace completes a full heat call without re-triggering E274.

When to call a professional

Call a licensed technician if the limit continues to trip after filter replacement and register checks, if manifold gas pressure is out of specification, or if ductwork modifications are needed to restore adequate airflow. A cracked heat exchanger can also cause persistent limit trips and is a carbon-monoxide hazard requiring combustion analysis and professional diagnosis.