Lennox SLP98UHV Error Code E 407: LSOM - Compressor Open Run Circuit
What Does Code E 407 Mean?
Through its LSOM (Lennox System Operation Monitor), the Lennox SLP98UHV control watches the outdoor compressor and shows faults on the furnace display. E 407 means the monitor found an open run circuit — the run winding or its path is not completing, so the compressor cannot sustain operation even if it manages to start.
This is commonly caused by a failed run winding inside the compressor or by the compressor's internal thermal protection opening after it has overheated. When the run circuit is open, the compressor may hum, start briefly and drop out, or not run at all.
E 407 is part of the LSOM compressor winding-and-electrical group. It is the run-side counterpart to E 406 (an open start circuit) and falls under the broader E 405 (a compressor open circuit). It differs from E 404, which is a mechanically locked rotor. A technician distinguishes an overheated compressor that can recover from a permanently open run winding through electrical testing after the compressor cools.
What You'll Notice
- The compressor tries to start but drops out and will not keep running
- No steady cooling; the vents blow warm air
- The outdoor unit hums briefly and then goes quiet
- The compressor is hot and cooling only works, if at all, in short bursts
- The outdoor fan runs while the compressor fails to stay on
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Compressor run winding failure or internal thermal protection tripped | Most common | ✗ Call a pro → |
How This Is Diagnosed
A technician confirms the unit has power, is set to cool, and is clear of debris — checks a homeowner can safely make. The electrical testing on the outdoor unit is the pro's responsibility.
Because overheating can open the run circuit through the internal protector, they let the compressor cool and recheck it. They then measure the run winding for continuity and verify refrigerant pressures and the outdoor fan. An open run winding after the compressor is cool indicates internal compressor failure and replacement; an overheating cause is corrected instead.
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 starts but immediately drops out and will not stay running
- Cooling only happens in brief bursts, if at all
- The compressor runs hot and the run circuit needs testing
- E 407 appears with E 405 or E 406, pointing to a compressor fault
Frequently Asked Questions
Why does my Lennox SLP98UHV compressor start then stop right away?
E 407 means the run circuit is open, so the compressor cannot sustain operation. It may start on the start winding but drop out because the run winding path is not completing.
Is E 407 fixable without replacing the compressor?
Sometimes. If the compressor merely overheated and tripped its internal protector, fixing the overheating cause can restore it. A permanently open run winding, however, means the compressor must be replaced.
How is E 407 different from E 406?
E 406 is an open start circuit (the compressor struggles to begin turning), while E 407 is an open run circuit (it cannot keep running). Both are LSOM compressor faults a technician tests for.
✓ Verified against manufacturer service manual — March 2026