Lennox SLP98UHV Error Code E 401: LSOM - Compressor Ran Over 18 Hours
What Does Code E 401 Mean?
Error code E401 on the Lennox SLP98UHV indicates that the air conditioning compressor has been running for more than 18 consecutive hours, triggering the Lennox Smart Outdoor Monitor (LSOM) protection. A compressor running this long without satisfying the thermostat setpoint signals a significant cooling system problem.
Prolonged compressor operation can be caused by low refrigerant levels (often from a leak), a severely dirty outdoor condenser coil, a failing compressor, or a system that is undersized for the cooling load. The compressor's internal protector may also have opened due to the extended run time, indicating overheating.
A technician should check the outdoor unit thoroughly — inspect the condenser coil for dirt buildup, verify the fan motor is operating, check electrical connections, and most importantly, test the refrigerant charge. If the system is low on refrigerant, there is likely a leak that must be found and repaired before recharging.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| AC system unable to reach thermostat setpoint due to low refrigerant or outdoor unit issue | Most common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- Your AC has been running all day without reaching the set temperature
- The outdoor unit is making unusual noises or the fan is not spinning
- You notice ice forming on the refrigerant lines or outdoor unit
✓ Verified against manufacturer service manual — March 2026