Lennox · Furnace error code

E240Low flame current during run

Cross-checked High-voltage / caution

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OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

Related diagnostic guides

Guide · Furnace

Flame sensor microamp test and cleaning (measuring flame signal)

Measure the actual flame current in series with a µA DC meter before and after cleaning. A clean sensor with a low reading points to ground, wiring, board or burner flame problems rather than the rod itself.

8 steps ~30 minmoderate

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Furnace short cycling (starts and stops quickly)

Short cycling is usually a safety opening mid-cycle (limit from low airflow, flame sensor dropping out, pressure switch opening) or a thermostat issue. Read the code and time the cycle.

4 steps ~45 minmoderate

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The flash code narrows the fault to one circuit before you open anything. Read it with power on, identify the board family, then use the door label to confirm. Only codes that two independent sources agree on are listed here.

8 steps ~10 mineasy