Lennox SLP98UHV Error Code E 409: LSOM - Compressor Low Voltage
What Does Code E 409 Mean?
Error code E409 on the Lennox SLP98UHV indicates that the Lennox Smart Outdoor Monitor (LSOM) has detected low voltage at the compressor. Running a compressor on low voltage is damaging — it causes the motor to draw excessive current to compensate for the reduced voltage, leading to overheating and premature failure.
Low voltage at the compressor can be caused by issues outside the HVAC system itself. Utility power problems, especially during peak summer demand, can drop the voltage below acceptable levels. Undersized electrical wiring between the breaker panel and the outdoor unit can cause voltage drop, particularly over long wire runs. A loose connection anywhere in the circuit — at the breaker, disconnect box, or contactor — can also cause voltage to drop under load.
An electrician or HVAC technician should measure the voltage at the outdoor unit disconnect under load (while the compressor is trying to run) and compare it to the compressor's rated voltage range. If the supply voltage is low, the electrical feed needs to be evaluated and corrected.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Low utility voltage or undersized electrical wiring to outdoor unit | Most common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- The outdoor unit shuts down on hot days when power demand is highest
- The compressor hums or struggles to start before shutting off
- You notice lights dimming in your home when the AC compressor tries to start
✓ Verified against manufacturer service manual — March 2026