Lennox SLP98UHV Error Code E 406: LSOM - Compressor Open Start Circuit
What Does Code E 406 Mean?
The Lennox SLP98UHV uses an LSOM (Lennox System Operation Monitor) to watch the outdoor compressor and report problems on the furnace display. E 406 means the monitor found an open start circuit — the winding or path the compressor uses to get started is not completing, so the motor cannot begin turning.
A very common reason is that the compressor has overheated and its internal thermal protector has opened, which will keep the start circuit open until the compressor cools down. Overheating itself can come from high head pressure, a weak outdoor fan, or refrigerant problems. An open start winding or a failed start component can also produce this fault.
E 406 belongs to the LSOM compressor winding-and-electrical group, alongside E 407 (an open run circuit) and the broader E 405 (a compressor open circuit). It is distinct from E 404 (a mechanically locked rotor). A technician will let the compressor cool, then check pressures, the outdoor fan, and the start components before deciding whether the compressor can recover or must be replaced.
What You'll Notice
- The outdoor compressor will not start, especially after running on a hot day
- Cooling stops and the vents blow warm air
- The compressor sometimes starts again after it has cooled for a while
- The outdoor unit feels very hot to the touch near the compressor
- The outdoor fan runs but the compressor stays off
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Compressor overheated and internal thermal protector tripped | Most common | ✗ Call a pro → |
How This Is Diagnosed
A technician confirms the unit has power and is set to cool, and checks that the outdoor unit is not blocked by debris — things a homeowner can safely verify. The rest is pro work on the outdoor equipment.
Because an overheated compressor commonly trips its internal protector, the technician lets the compressor cool down and then rechecks the start circuit. They also measure refrigerant pressures and confirm the outdoor fan is moving air, since overheating often traces back to those. If the start circuit stays open after the compressor is cool, the compressor has failed internally.
When to Call a Professional
This code involves components that are not homeowner-serviceable, so have a licensed HVAC technician diagnose and repair it. Keep in mind:
- The compressor stops starting after running on a hot day, then recovers when cool
- The outdoor unit runs very hot and cooling has become unreliable
- The compressor will not restart even after it has cooled down
- Refrigerant pressures and the outdoor fan need to be checked by a pro
Frequently Asked Questions
Why does my Lennox SLP98UHV compressor start again after cooling off?
E 406 often means the compressor overheated and its internal protector opened the start circuit. Once the compressor cools, the protector can reset and it may start again — but the overheating cause still needs to be found.
What makes the compressor overheat and trip E 406?
High head pressure, a weak or failed outdoor fan, low airflow, or refrigerant problems can all overheat the compressor. A technician checks pressures and the fan to find the root cause.
Is E 406 different from E 405?
Yes. E 405 is a general compressor open circuit, while E 406 specifically points to the start circuit being open, commonly from an overheated compressor tripping its thermal protector.
✓ Verified against manufacturer service manual — March 2026