Lennox SLP98UHV Error Code E 409: LSOM - Compressor Low Voltage
What Does Code E 409 Mean?
The Lennox SLP98UHV control uses an LSOM (Lennox System Operation Monitor) to watch the outdoor compressor and report its condition on the furnace. E 409 means the monitor detected low voltage reaching the compressor. When supply voltage sags, the compressor motor draws more current, runs hotter, and may struggle to start or stay running.
Low voltage can come from the utility side (a brown-out or generally weak service), from undersized or overloaded wiring to the outdoor unit, or from a loose or corroded connection at the disconnect or contactor that drops voltage under load. Left uncorrected, chronic low voltage can overheat and eventually damage the compressor windings.
E 409 is part of the compressor supply-and-contactor group within the LSOM family, alongside E 408 (a welded compressor contactor). Both concern the power feeding the compressor rather than its internal windings, which are covered by E 405, E 406, and E 407. Because it involves line-voltage testing at the outdoor unit, this is not a homeowner repair.
What You'll Notice
- The compressor struggles to start or hums before starting during cooling calls
- Cooling is weak or intermittent, especially during peak electrical demand
- Lights in the home dim when the outdoor unit tries to start
- The outdoor unit runs hot or cycles off on its protector
- Cooling problems coincide with brown-outs or heavy neighborhood power use
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Low utility voltage or undersized electrical wiring to outdoor unit | Most common | ✗ Call a pro → |
How This Is Diagnosed
A technician confirms the outdoor unit has power and is set to cool — a homeowner can safely verify the thermostat and that the disconnect is on. The voltage testing that follows is pro work on the outdoor equipment.
They measure the line voltage at the outdoor unit disconnect while the compressor is trying to start and run. If voltage sags below spec, they trace whether it is a utility-side problem, undersized wiring, or a loose or corroded connection at the disconnect or contactor, then correct the source so the compressor receives full voltage.
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 hums or struggles to start, especially during high power demand
- Home lights dim when the outdoor unit tries to start
- Cooling problems line up with brown-outs or weak utility power
- The line voltage at the outdoor unit disconnect needs to be measured
Frequently Asked Questions
What causes low voltage to my Lennox SLP98UHV outdoor unit?
E 409 can come from weak utility power or a brown-out, undersized wiring to the outdoor unit, or a loose or corroded connection at the disconnect or contactor. A technician measures the voltage to find the source.
Can low voltage damage my compressor?
Yes. Running on low voltage makes the motor draw more current and run hotter, which over time can overheat and damage the compressor windings. That is why E 409 should be corrected promptly.
Why do my lights dim when the AC starts with E 409?
Dimming lights suggest the voltage is sagging when the compressor draws its starting current, which is consistent with a low-voltage or weak-supply condition that a technician or electrician should check.
✓ Verified against manufacturer service manual — March 2026