Lennox SLP98UHV Error Code E 402: LSOM - Outdoor Unit Pressure Trip
What Does Code E 402 Mean?
Through its LSOM (Lennox System Operation Monitor) function, the Lennox SLP98UHV control watches the outdoor compressor and shows its status on the furnace's 7-segment display. E 402 means the outdoor unit tripped a pressure safety switch — either a high-pressure or low-pressure limit — and the compressor was shut off to protect it.
A high-pressure trip usually points to poor heat rejection: a dirty condenser coil, a failed or weak outdoor fan, or blocked airflow around the unit. A low-pressure trip usually points to a loss of refrigerant from a leak, or a restriction in the refrigerant lines. In both cases the safety switch is doing its job by stopping the compressor before damage occurs.
E 402 is closely tied to E 401 (the compressor running more than 18 hours without meeting the load) — both point back to refrigerant charge and airflow at the outdoor unit. It is also part of the same LSOM family as E 403 (the compressor short-cycling, running less than about four minutes per cycle). Because refrigerant work is involved, this is not a homeowner repair.
What You'll Notice
- The outdoor unit shuts off shortly after starting a cooling cycle
- Cooling stops entirely and warm air comes from the vents
- The outdoor coil is visibly dirty or clogged with debris (high-pressure side)
- Frost or ice forms on the refrigerant lines or indoor coil (low-pressure side)
- The system cycles off repeatedly and never delivers steady cooling
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| High or low refrigerant pressure causing safety trip | Most common | ✗ Call a pro → |
How This Is Diagnosed
A technician first rules out the homeowner-safe causes: an outdoor coil blocked by debris and a fan that is not turning are easy to spot. Confirming the unit is clear and the thermostat is set to cool is fine for a homeowner to check; the pressure and refrigerant work is not.
They connect gauges to read the high and low side pressures, which tells them whether the trip was high-pressure (typically dirty coil, weak fan, restricted airflow) or low-pressure (typically low refrigerant from a leak, or a line restriction). The coil is cleaned and airflow restored for a high-side trip; a low-side trip leads to a leak search and repair before recharging.
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 outdoor unit trips off soon after starting a cooling cycle
- The condenser coil is dirty or the outdoor fan is not spinning
- Ice or frost forms on the refrigerant lines or indoor coil
- E 402 appears with E 401, pointing to a refrigerant or airflow issue
- System pressures and refrigerant charge need to be measured
Frequently Asked Questions
What does an outdoor pressure trip mean on my Lennox SLP98UHV?
E 402 means a high- or low-pressure safety switch on the outdoor unit opened and stopped the compressor. High pressure usually means poor heat rejection (dirty coil, weak fan); low pressure usually means lost refrigerant.
Can I reset E 402 and keep using the AC?
You can restore power, but if the underlying refrigerant or airflow problem is not fixed the switch will just trip again. Repeated trips stress the compressor, so it is best to have it diagnosed.
Why is there ice on my refrigerant line with E 402?
Icing often accompanies a low-pressure (low-refrigerant) trip. Low charge drops the coil temperature below freezing, and a technician needs to find the leak and recharge the system.
✓ Verified against manufacturer service manual — March 2026