Lennox SLP98UHV Error Code E 408: LSOM - Compressor Contactor Welded
What Does Code E 408 Mean?
The Lennox SLP98UHV control uses an LSOM (Lennox System Operation Monitor) to watch the outdoor compressor and display its condition on the furnace. E 408 means the outdoor unit's contactor — the electrical switch that connects power to the compressor — has welded shut. Its contacts have fused together, so the circuit stays closed even when the control is no longer calling for cooling.
A welded contactor is a real problem because the compressor keeps receiving power and can run continuously, which wastes energy and can overheat or damage the compressor over time. Contactors weld when their contacts arc repeatedly and the resulting heat fuses the metal, often after years of normal switching.
E 408 is part of the compressor supply-and-contactor group within the LSOM family, alongside E 409 (compressor low voltage). Both involve the power feeding the compressor rather than its internal windings, unlike E 405, E 406, and E 407, which are winding faults. Replacing a welded contactor is electrical work at the outdoor unit and must be done by a technician.
What You'll Notice
- The outdoor compressor keeps running even when the thermostat is satisfied or off
- The AC will not shut off and the home gets colder than the setpoint
- The outdoor unit runs almost constantly and electricity use rises
- The compressor runs even with the thermostat set to heat or off
- The outdoor unit feels hot from running without a break
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Electrical arcing welded the contactor contacts shut | Most common | ✗ Call a pro → |
How This Is Diagnosed
If the compressor is running continuously and will not shut off, a homeowner can safely turn off power to the outdoor unit at its disconnect box or breaker to stop it until a technician arrives. The contactor work itself is not a DIY task.
The technician confirms the contactor is stuck closed by checking whether the compressor still has power with no cooling call present. A welded contactor is replaced with the correct part, after which they verify the compressor cycles on and off normally with the thermostat.
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 runs continuously and will not shut off — turn off power to the outdoor unit at the disconnect or breaker until it is repaired
- The AC keeps cooling past the setpoint or runs with the thermostat off
- The outdoor unit runs without a break and gets very hot
- The welded contactor at the outdoor unit needs to be replaced
Frequently Asked Questions
My Lennox SLP98UHV AC will not shut off — what should I do?
E 408 means the outdoor contactor has welded shut, so the compressor keeps getting power. You can safely turn off power to the outdoor unit at its disconnect or breaker to stop it, then call a technician to replace the contactor.
Is it safe to let the compressor keep running with E 408?
No. Running continuously wastes energy and can overheat or damage the compressor. Shutting off power at the outdoor disconnect until it is repaired protects the equipment.
Why did the contactor weld shut?
Contactors switch high current, and over years the contacts arc and heat up. Eventually that heat can fuse the contacts together so they no longer open, which is what E 408 reports.
✓ Verified against manufacturer service manual — March 2026