Lennox SLP98UHV Error Code E 406: LSOM - Compressor Open Start Circuit
What Does Code E 406 Mean?
Error code E406 on the Lennox SLP98UHV indicates that the Lennox Smart Outdoor Monitor (LSOM) has detected an open start circuit in the compressor. The start circuit is responsible for getting the compressor motor spinning during startup, and an open circuit means current cannot flow through this winding.
The most common cause is a compressor that has overheated and tripped its internal thermal protector. When the compressor gets too hot — due to low refrigerant, dirty coils, a failed fan motor, or prolonged operation — the thermal protector opens the start circuit to prevent further damage. After the compressor cools down (which can take several hours), the protector may reset and allow the compressor to restart.
If the compressor does not restart after cooling, the start winding may have permanently failed. A technician should allow the compressor to cool completely, then check refrigerant pressures, verify the condenser fan is operating, and test the compressor motor windings. If the start winding is open with the compressor cool, the compressor needs replacement.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Compressor overheated and internal thermal protector tripped | Most common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- The compressor does not restart after being allowed to cool for several hours
- The outdoor unit shuts down on hot days and E406 appears
- You hear the compressor try to start with a buzzing sound before shutting off
✓ Verified against manufacturer service manual — March 2026