Lennox E407

What does Lennox E407 mean?

E407 is a Service Urgent LSOM alert indicating insufficient current through the compressor RUN current transformer. The run winding carries the majority of the compressor's operating current; when the LSOM detects its absence during a compressor demand, E407 fires. The alert clears after current is sensed in the RUN sensor, after one normal compressor run cycle, or after a power reset.

Symptoms

• E407 or 'OD Compressor Open Run Circuit' shown on the thermostat.
• Compressor does not sustain operation — may start briefly then shut down.
• No steady current draw from the compressor despite the contactor being closed.
• System fails to cool or heat; outdoor unit appears to cycle on and off rapidly.

Common causes

• Open run winding in the compressor motor.
• Failed run capacitor causing the run winding to lose current mid-cycle.
• Broken wire or loose connection at the R (run) terminal on the compressor.
• Compressor internal overload tripping on the run winding due to overheating.
• Contactor contact erosion preventing sustained current flow at the run winding terminal.

Diagnostic steps

1. 01

   Test the run capacitor

   With power off and the capacitor discharged, check capacitance against the nameplate value. A weak or open run capacitor is the most common external cause of a run-circuit open condition. Replace if out of specification.

2. 02

   Inspect run-terminal wiring

   Disconnect power and check the R (run) spade connector on the compressor terminal block for looseness, corrosion, or heat damage. A loose connection that opens under current is indistinguishable from an open winding until wiring is inspected.

3. 03

   Measure run winding resistance

   With an ohmmeter across C and R terminals, compare resistance to specification. An open (infinite) reading confirms a failed run winding requiring compressor replacement.

4. 04

   Check refrigerant pressures and fan motor operation

   Per Lennox documentation, verify operating pressures, condenser fan motor function, and refrigerant condition. High return gas temperature or liquid flooding can cause the run winding overload to trip during operation.

5. 05

   Replace compressor if winding is confirmed open

   If the run winding resistance is open and all external wiring and capacitor checks are good, the compressor requires replacement. Do not continue to attempt restarts — each locked-rotor attempt risks additional damage.

When to call a professional

Compressor winding failure diagnosis and replacement requires refrigerant recovery, system flushing (especially after a burnout), nitrogen pressure testing, deep evacuation, and accurate recharge with verified subcooling and superheat. These tasks require a licensed HVAC technician with EPA Section 608 certification and appropriate tools.