Lennox E406 Error Code

What does Lennox E406 mean?

E406 is a Service Urgent LSOM alert indicating insufficient current through the compressor START current transformer. The LSOM monitors both start and run windings separately via current transformers; E406 fires when start-winding current is absent or too low when the compressor is commanded on. The alert clears after current is sensed in the START sensor or after a power reset.

Symptoms

• E406 or 'OD Compressor Open Start Circuit' shown on the thermostat.
• Compressor may hum or make a brief noise but fails to start.
• Run winding may draw current briefly before the overload trips.
• System does not cool or heat despite the contactor closing.

Common causes

• Failed start capacitor (if equipped) — open start capacitor prevents start-winding current.
• Open start winding in the compressor motor.
• Broken wire or loose connector on the start-winding terminal.
• Compressor too hot to start — internal thermal overload on start winding has tripped.
• Start relay contacts not closing to connect the start capacitor at startup.

Ask the AskWhiz AI about Lennox E406

AskWhiz

online

Hi — I'm the HVAC demo. Ask me anything in this domain.

What causes Lennox E406?Step-by-step fix for Lennox E406?Is Lennox E406 safe to ignore?

MessageHelp me troubleshoot Lennox E406

Diagnostic steps

1. Allow the compressor to cool down

   If the compressor has been operating or attempting to start repeatedly, allow 45–90 minutes for the internal overload to cool and reset before making any measurements.

2. Inspect start and run capacitor

   With power off and capacitors safely discharged, test capacitance of both the run and start capacitors (if a start capacitor/relay kit is installed). A failed start capacitor prevents start-winding current from flowing.

3. Check start-winding terminal connections

   Disconnect compressor terminals and inspect the S (start) terminal spade for a broken wire, corroded connector, or heat damage. Reconnect securely if loose.

4. Measure start winding resistance

   With an ohmmeter, measure resistance from S to C on the compressor terminal block. Compare to specification. An open reading (infinite resistance) indicates a failed start winding requiring compressor replacement.

5. Check refrigerant pressure and operating conditions

   Per Lennox documentation, check operating pressures, fan motor operation, and refrigerant state. High discharge pressure at startup can prevent the start winding from accelerating the compressor, mimicking an open start circuit.

When to call a professional

A confirmed open compressor start winding requires compressor replacement. If the start circuit checks good externally but E406 persists, a licensed technician with a clamp meter and refrigerant manifold set must evaluate system pressures, electrical supply quality, and compressor performance under load. Compressor replacement requires EPA-certified refrigerant handling.