Lennox E402 Error Code

What does Lennox E402 mean?

E402 is a Service Soon alert from the LSOM indicating an outdoor unit system pressure trip — either discharge or suction pressure is out of limits, or the compressor has overloaded. The LSOM detects this after the compressor ran continuously for over 18 hours in cooling mode. The alert clears after four consecutive normal compressor run cycles.

Symptoms

• E402 or 'OD System Pressure Trip' shown on the thermostat.
• Outdoor unit may shut down on high or low pressure safety.
• Cooling capacity is reduced or the system fails to maintain setpoint.
• Alert recurs if the underlying pressure condition is not corrected.

Common causes

• Dirty condenser coil causing high discharge pressure.
• Low refrigerant charge causing low suction pressure and compressor overload.
• Condenser fan failure reducing heat rejection and driving up head pressure.
• Refrigerant overcharge causing high discharge pressure.
• Compressor overloading due to high return gas temperature or liquid refrigerant slugging.

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Diagnostic steps

1. Connect manifold gauges and check operating pressures

   Measure suction and discharge pressure at the service ports. Compare to the manufacturer pressure-temperature chart for the refrigerant type and current conditions. Out-of-range readings confirm a refrigerant charge or mechanical issue.

2. Inspect and clean the condenser coil

   Turn off power at the disconnect and inspect condenser fins. Clean with a water hose spraying from inside out. A fouled condenser coil raises discharge pressure above the trip point.

3. Confirm condenser fan operation

   Verify the condenser fan starts on cooling demand and rotates in the correct direction. Check fan motor amperage against the nameplate. A failed fan causes rapid pressure rise.

4. Check refrigerant charge

   With manifold gauges installed, evaluate subcooling (liquid line) and superheat (suction line) against target values for the system. Adjust charge only after verifying there are no leaks.

When to call a professional

Pressure-related compressor trips require refrigerant system diagnosis with manifold gauges, which demands EPA Section 608 certification. Refrigerant charge adjustment, leak repair, and condenser fan motor replacement all require a licensed technician. Do not operate the system with known pressure trip conditions — repeated high-pressure trips can cause compressor valve damage.