Lennox E404

What does Lennox E404 mean?

E404 is a Service Urgent alert from the LSOM indicating a locked rotor condition in the compressor. The compressor motor cannot start or accelerate to running speed. This is caused by a failed run capacitor, seized bearings, or liquid refrigerant flooding the crankcase. The alert clears after four consecutive normal compressor run cycles or a power reset.

Symptoms

• E404 or 'OD Compressor Rotor Locked' shown on the thermostat.
• Outdoor unit hums loudly but compressor does not start — contactor pulls in but compressor stalls.
• Circuit breaker or fuse may trip due to high locked-rotor amperage.
• Compressor internal overload trips shortly after startup attempt.

Common causes

• Failed run capacitor preventing the motor from developing starting torque.
• Seized compressor bearings from loss of lubrication or contamination.
• Liquid refrigerant in the crankcase causing hydraulic lock at startup.
• High discharge pressure from a flooded condenser preventing the motor from overcoming back-pressure.
• Worn compressor motor windings that cannot produce sufficient torque.

Diagnostic steps

1. 01

   Disconnect power and test the run capacitor

   With power off and the capacitor discharged, test capacitance with a meter. Compare to the nameplate value (typically within ±6%). A weak or failed capacitor prevents starting torque — replace with the correct MFD and voltage rating.

2. 02

   Check for liquid refrigerant in the crankcase

   If the system was off for an extended period or experienced a refrigerant leak and recharge, liquid may have migrated into the crankcase. Energize the crankcase heater (if equipped) for several hours before attempting a restart.

3. 03

   Install a hard-start kit if capacitor checks good

   A hard-start capacitor/relay kit provides additional starting torque for a compressor with mechanical resistance. Lennox documentation recommends considering a hard-start kit when rotor locked conditions occur without clear capacitor failure.

4. 04

   Measure compressor winding resistance

   With power off, disconnect compressor terminals and measure resistance between C-S, C-R, and S-R. Compare to specification. An open winding or reading outside tolerance confirms compressor motor failure.

When to call a professional

A confirmed locked-rotor compressor with failed windings or seized bearings requires compressor replacement — a job requiring refrigerant recovery, system flushing if contamination is present, nitrogen pressure testing, deep evacuation, and accurate recharge. This is licensed technician work in all jurisdictions. Repeatedly attempting to start a locked rotor compressor without diagnosis will cause electrical damage to the control board and wiring.