ErrorCodePro · Appliance Error CodesLennox AC & Heat Pump Error Codes: Full List & Meanings
Every Lennox ac & heat pump error code we have verified against Lennox's own support pages, with its official meaning. This list covers outdoor unit alerts (lsom) shown on icomfort-communicating sl280dfv systems. Codes vary by model, so confirm in your owner's manual. Support: Lennox dealer (lennox.com).
Codes by system
Power supply
All Lennox ac & heat pump error codes
Codes marked "Reset only" or "Service" have no separate page because the manufacturer publishes no owner fix beyond a reset or a service call.
| Code | Official meaning | System | Owner action |
|---|---|---|---|
| E400 | There's a cooling demand but the compressor isn't running. Lennox says to check power to the outdoor unit. | Power supply | Owner checks |
| E405 | The compressor circuit is open, for example from a power disconnection or open fuse. | Power supply | Owner checks |
| E401 | The manufacturer lists E401 as: compressor long run cycle (over 18 hours) or low system pressure (critical alert). A licensed HVAC technician must diagnose this. | Heating | Service |
| E402 | The manufacturer lists E402 as: outdoor unit system pressure trip. A licensed HVAC technician must diagnose this. | Heating | Service |
| E403 | The manufacturer lists E403 as: compressor short-cycling. A licensed HVAC technician must diagnose this. | Heating | Service |
| E404 | The manufacturer lists E404 as: compressor rotor locked. A licensed HVAC technician must diagnose this. | Motor / drive | Service |
| E406 | The manufacturer lists E406 as: compressor open start circuit. A licensed HVAC technician must diagnose this. | Control / electronics | Service |
| E407 | The manufacturer lists E407 as: compressor open run circuit. A licensed HVAC technician must diagnose this. | Control / electronics | Service |
| E408 | The manufacturer lists E408 as: compressor contactor welded (runs continuously). A licensed HVAC technician must diagnose this. | Control / electronics | Service |
| E409 | The manufacturer lists E409 as: compressor low voltage (24 VAC below 18 V). A licensed HVAC technician must diagnose this. | Power supply | Service |
Symptoms
Sources
Last reviewed: October 20, 2026. ErrorCodePro is not affiliated with Lennox.