Lennox SLP98UHV Error Code E 408: LSOM - Compressor Contactor Welded
What Does Code E 408 Mean?
Error code E408 on the Lennox SLP98UHV indicates that the Lennox Smart Outdoor Monitor (LSOM) has detected a welded compressor contactor. The contactor is an electrically operated switch that turns the compressor on and off. When its contacts weld (fuse) together, the compressor can run continuously and cannot be turned off by normal control signals.
Contactor welding occurs when electrical arcing gradually erodes and melts the contact surfaces until they fuse together. This typically happens over time as the contactor ages, but can be accelerated by low voltage conditions, oversized compressor loads, or a failing contactor coil that causes chattering.
This is a safety concern because the compressor will run uncontrolled until power is physically disconnected. Turn off the circuit breaker for the outdoor unit until a technician can replace the contactor. The replacement is a relatively straightforward and inexpensive repair — the contactor is a standard, replaceable component.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Electrical arcing welded the contactor contacts shut | Most common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- The compressor runs constantly even when the thermostat is not calling for cooling
- The outdoor unit does not shut off when expected
- Turn off the outdoor unit's circuit breaker and schedule service immediately
✓ Verified against manufacturer service manual — March 2026