Lennox Furnace Error Code E409

E409 indicates that the secondary control voltage in your Lennox furnace has dropped below 18VAC, triggering a low voltage fault on the LSOM (Low Secondary Output Module). After operating under these low-voltage conditions for 10 minutes, the furnace shuts down to protect the compressor and control components. The code will clear automatically once the secondary voltage rises above 20VAC for at least 2 seconds, or after a manual power reset.

The furnace is designed to protect itself by shutting down after 10 minutes when this fault is active, so it will not run indefinitely under unsafe conditions. However, repeatedly allowing the system to cycle through this fault can cause additional stress on the compressor and control board over time. It is best to have the low-voltage circuit inspected and repaired promptly rather than continuing to reset the unit.

Start by measuring the secondary voltage at the transformer output terminals while the system is running and all control loads are connected. A reading consistently below 18VAC confirms the fault is genuine. From there, check for overloaded circuits by disconnecting accessories one at a time, inspect all low-voltage wiring for shorts or loose connections, and verify that the incoming line voltage to the transformer primary is within the normal range. If the transformer output is low even with minimal load, the transformer itself is likely the component to replace.

Yes, a power reset will clear the E409 code, and it will also clear automatically if the secondary voltage recovers above 20VAC for 2 seconds. However, if the root cause of the low voltage condition is not identified and corrected, the fault will return and the furnace will shut down again after 10 minutes of operation. It is important to diagnose and repair the underlying voltage issue rather than relying on resets alone.

The repair cost depends heavily on the root cause. If the fix is simply tightening a loose connection or correcting a wiring short, labor costs may be in the range of $75 to $150. If the control transformer needs replacement, parts and labor combined typically run between $150 and $350. More complex issues involving the control board or extensive wiring repairs could push costs higher.