Lennox SLP98UHV Error Code E 405: LSOM - Compressor Open Circuit
What Does Code E 405 Mean?
The Lennox SLP98UHV control includes an LSOM (Lennox System Operation Monitor) that watches the outdoor compressor and reports faults on the furnace display. E 405 means the monitor sees an open circuit at the compressor — the electrical path through the compressor and its starting components is broken, so the motor cannot draw power to run.
Common causes include a failed run capacitor, broken or loose wiring between the contactor and compressor, or an open motor winding inside the compressor itself. On this outdoor unit the capacitor is one of the more frequently failed parts, so it is checked early, but an open winding points to a failing compressor.
E 405 is part of the LSOM compressor-fault family. It is closely related to E 404 (the compressor rotor being mechanically locked) and to the more specific winding faults E 406 (an open start circuit) and E 407 (an open run circuit). A technician distinguishes a simple capacitor or wiring failure from an internal compressor failure through electrical testing.
What You'll Notice
- The outdoor unit does not start when the thermostat calls for cooling
- No cooling; the supply vents blow warm or room-temperature air
- The outdoor fan may run while the compressor stays silent
- A humming or clicking at the contactor without the compressor starting
- The system worked intermittently before failing to start at all
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Failed compressor motor winding or bad run capacitor | Most common | ✗ Call a pro → |
How This Is Diagnosed
The technician confirms the unit has power and is set to cool — checks a homeowner can safely make — then moves to electrical testing, which is not a DIY task on this outdoor equipment.
They test the run capacitor first, since a failed capacitor is a common and simple cause of an open-circuit reading. If the capacitor and wiring are good, they measure the compressor windings for continuity. An open winding means the compressor has failed internally and needs replacement; a broken wire or bad connection is repaired 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 outdoor compressor will not start on a cooling call
- The contactor hums or clicks but the compressor stays silent
- The run capacitor or compressor wiring needs professional testing
- E 405 appears with E 404, E 406, or E 407, indicating a compressor fault
Frequently Asked Questions
Could a bad capacitor cause E 405 on my Lennox SLP98UHV?
Yes. A failed run capacitor is one of the most common reasons the compressor shows an open circuit and will not start. A technician tests the capacitor before condemning the compressor.
Does E 405 mean my compressor is dead?
Not necessarily. The open circuit can come from a capacitor or wiring problem outside the compressor. Only after those test good and a winding reads open is the compressor itself the problem.
Why does the outdoor fan run but there is no cooling?
The fan and compressor are separate. With E 405 the compressor cannot start due to an open circuit, so the fan may spin while no cooling is produced.
✓ Verified against manufacturer service manual — March 2026