Lennox Furnace Error Code E406

E406 on a Lennox furnace indicates that the LSOM (Lennox System Operation Monitor) has detected an open start circuit on the compressor. The control board monitors current through a Start current transformer and triggers this fault when the required current is not detected during a compressor startup attempt. This fault will clear once current is properly sensed in the START sensor, or after a power reset is performed.

When E406 is active, the system has already halted compressor operation to prevent damage, so the compressor is not running. Continuing to attempt restarts without diagnosing the root cause risks damaging the compressor further, particularly if the start winding is drawing excessive current during failed start attempts. It is best to stop cycling the unit and have the system inspected promptly.

Repair costs vary significantly depending on the failed component. Replacing a start capacitor is generally an inexpensive repair, typically ranging from $75 to $200 including labor. A start relay replacement falls in a similar range. If the compressor itself has a failed start winding, replacement costs can range from $1,000 to $2,500 or more depending on unit size and refrigerant type, making a full system replacement worth considering in those cases.

Some basic checks, such as inspecting wiring connections at the compressor terminals for looseness or corrosion, can be done by a knowledgeable homeowner. However, testing and replacing components like start capacitors, relays, or current transformers involves working with high-voltage circuits and should be performed by a qualified HVAC technician. Attempting repairs without proper training and equipment carries serious safety risks.

A power reset will clear the E406 fault code, and the code will also clear automatically if current is properly detected in the START sensor circuit on the next startup attempt. However, if the underlying problem is not corrected, the fault will return when the compressor attempts to start again. Clearing the code without diagnosing and repairing the cause is only a temporary measure.