Lennox SLP98UHV Error Code E 402: LSOM - Outdoor Unit Pressure Trip
What Does Code E 402 Mean?
Error code E402 on the Lennox SLP98UHV indicates that the outdoor unit has experienced a pressure trip, detected by the Lennox Smart Outdoor Monitor (LSOM). A high or low-pressure safety switch in the outdoor unit has opened, shutting down the compressor to prevent damage.
High-pressure trips typically occur when the outdoor condenser coil is extremely dirty, the condenser fan motor has failed, or the refrigerant system is overcharged. The condenser cannot reject heat effectively, causing refrigerant pressure to spike above safe levels. Low-pressure trips occur when the system is low on refrigerant (usually from a leak), the indoor evaporator coil is frozen, or there is a restriction in the refrigerant lines.
A licensed HVAC technician should inspect the outdoor unit, clean the condenser coils, verify the fan motor is operating, and test the refrigerant pressures on both the high and low sides of the system. If the refrigerant charge is incorrect, the source of the problem must be identified and corrected.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| High or low refrigerant pressure causing safety trip | Most common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- The outdoor unit shuts down and the furnace displays E402
- You notice the outdoor unit's fan is not running
- There is ice on the refrigerant lines or either coil
✓ Verified against manufacturer service manual — March 2026