Lennox E405

What does Lennox E405 mean?

E405 is a Service Urgent alert from the LSOM indicating the compressor circuit is open — no current is detected in the compressor circuit when the contactor is closed. The LSOM detects this via its current transformers and flags the condition immediately. The alert clears after one normal compressor run cycle.

Symptoms

• E405 or 'OD Compressor Open Circuit' shown on the thermostat.
• Outdoor unit contactor pulls in but compressor does not start and draws no current.
• No hum or vibration from the compressor terminal box.
• Cooling or heating demand not satisfied despite the outdoor unit appearing to energize.

Common causes

• Open power disconnect at the outdoor unit.
• Blown fuse or tripped circuit breaker in the compressor circuit.
• Broken wire or a connector not making contact in the compressor circuit.
• Open compressor internal overload protector (compressor too hot to reset).
• Failed compressor contactor contacts not closing despite the coil energizing.

Diagnostic steps

1. 01

   Verify outdoor unit power disconnect is closed

   Confirm the pull-out or switch-type disconnect at the outdoor unit is fully seated and its fuses are intact. An open disconnect is the simplest cause of E405.

2. 02

   Check circuit breaker and fuses

   At the electrical panel, confirm the breaker serving the outdoor unit is in the ON position and has not tripped. Check all fuses in the outdoor disconnect for continuity with a meter.

3. 03

   Inspect compressor wiring and connections

   With power off, inspect the compressor terminal block for loose spade connectors, broken wires, or heat damage. Tug each terminal individually — a loose connection under load will cause the LSOM to detect an open circuit.

4. 04

   Test contactor contacts

   With a voltmeter, check for line voltage at the load side of the contactor when the contactor coil is energized. If the coil pulls in but load-side voltage is absent, the contactor contacts are open or burned. Replace the contactor.

5. 05

   Allow the compressor to cool and retest

   If the compressor internal overload has tripped, the compressor body will be very hot. Allow 45–90 minutes for the overload to cool and reset. Retest for current flow after reset.

When to call a professional

If all external circuit checks pass and the compressor still draws no current, the compressor motor has an open winding and requires replacement. Compressor replacement involves refrigerant recovery, system decontamination if a burnout has occurred, deep evacuation, and recharge — all requiring EPA Section 608 certification and specialized equipment.